From ba434981e3ccc4bd0e1a929d0e70982de9ef4152 Mon Sep 17 00:00:00 2001
From: Anna Gringauze <annagrin@google.com>
Date: Wed, 13 Jul 2022 11:38:27 -0700
Subject: [PATCH 1/7] Migrate debugging/inspector.dart to null safety

---
 dwds/lib/src/debugging/debugger.dart          |   6 +-
 dwds/lib/src/debugging/inspector.dart         | 142 ++++++++++--------
 dwds/lib/src/debugging/instance.dart          |   3 +
 .../src/services/chrome_proxy_service.dart    |   4 +-
 dwds/lib/src/utilities/domain.dart            |  10 +-
 dwds/lib/src/utilities/shared.dart            |  50 ++++--
 dwds/test/inspector_test.dart                 |  38 +++++
 7 files changed, 170 insertions(+), 83 deletions(-)

diff --git a/dwds/lib/src/debugging/debugger.dart b/dwds/lib/src/debugging/debugger.dart
index a8f226e3f..9ab72eb76 100644
--- a/dwds/lib/src/debugging/debugger.dart
+++ b/dwds/lib/src/debugging/debugger.dart
@@ -12,7 +12,6 @@ import 'package:webkit_inspection_protocol/webkit_inspection_protocol.dart'
     hide StackTrace;
 
 import '../loaders/strategy.dart';
-import '../services/chrome_debug_exception.dart';
 import '../utilities/conversions.dart';
 import '../utilities/dart_uri.dart';
 import '../utilities/domain.dart';
@@ -722,12 +721,9 @@ class Debugger extends Domain {
     try {
       return await _remoteDebugger.evaluateOnCallFrame(callFrameId, expression);
     } on ExceptionDetails catch (e) {
-      throw ChromeDebugException(
+      throwChromeDebugException(
         e.json,
         evalContents: expression,
-        additionalDetails: {
-          'Dart expression': expression,
-        },
       );
     }
   }
diff --git a/dwds/lib/src/debugging/inspector.dart b/dwds/lib/src/debugging/inspector.dart
index dc4d50228..080dfb1be 100644
--- a/dwds/lib/src/debugging/inspector.dart
+++ b/dwds/lib/src/debugging/inspector.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.import 'dart:async';
 
-// @dart = 2.9
-
 import 'package:async/async.dart';
 import 'package:logging/logging.dart';
 import 'package:vm_service/vm_service.dart';
@@ -67,9 +65,9 @@ class AppInspector implements AppInspectorInterface {
 
   final ExecutionContext _executionContext;
 
-  LibraryHelper _libraryHelper;
-  ClassHelper _classHelper;
-  InstanceHelper _instanceHelper;
+  late final LibraryHelper _libraryHelper;
+  late final ClassHelper _classHelper;
+  late final InstanceHelper _instanceHelper;
 
   final AssetReader _assetReader;
   final Locations _locations;
@@ -106,15 +104,16 @@ class AppInspector implements AppInspectorInterface {
 
     final libraries = await _libraryHelper.libraryRefs;
     isolate.rootLib = await _libraryHelper.rootLib;
-    isolate.libraries.addAll(libraries);
+    isolate.libraries?.addAll(libraries);
 
     final scripts = await scriptRefs;
 
     await DartUri.initialize(_sdkConfiguration);
-    await DartUri.recordAbsoluteUris(libraries.map((lib) => lib.uri));
-    await DartUri.recordAbsoluteUris(scripts.map((script) => script.uri));
+    await DartUri.recordAbsoluteUris(libraries.map((lib) => lib.uri).cast());
+    await DartUri.recordAbsoluteUris(
+        scripts.map((script) => script.uri).cast());
 
-    isolate.extensionRPCs.addAll(await _getExtensionRpcs());
+    isolate.extensionRPCs?.addAll(await _getExtensionRpcs());
   }
 
   static IsolateRef _toIsolateRef(Isolate isolate) => IsolateRef(
@@ -187,7 +186,7 @@ class AppInspector implements AppInspectorInterface {
 
   /// Returns the ID for the execution context or null if not found.
   @override
-  Future<int> get contextId async {
+  Future<int?> get contextId async {
     try {
       return await _executionContext.id;
     } catch (e, s) {
@@ -236,15 +235,16 @@ class AppInspector implements AppInspectorInterface {
       String evalExpression, List<RemoteObject> arguments,
       {bool returnByValue = false}) async {
     final jsArguments = arguments.map(callArgumentFor).toList();
-    final result =
+    final response =
         await remoteDebugger.sendCommand('Runtime.callFunctionOn', params: {
       'functionDeclaration': evalExpression,
       'arguments': jsArguments,
       'objectId': receiver.objectId,
       'returnByValue': returnByValue,
     });
-    handleErrorIfPresent(result, evalContents: evalExpression);
-    return RemoteObject(result.result['result'] as Map<String, Object>);
+    final result =
+        getResultOrHandleError(response, evalContents: evalExpression);
+    return RemoteObject(result);
   }
 
   /// Calls Chrome's Runtime.callFunctionOn method with a global function.
@@ -255,15 +255,16 @@ class AppInspector implements AppInspectorInterface {
       String evalExpression, List<Object> arguments,
       {bool returnByValue = false}) async {
     final jsArguments = arguments.map(callArgumentFor).toList();
-    final result =
+    final response =
         await remoteDebugger.sendCommand('Runtime.callFunctionOn', params: {
       'functionDeclaration': evalExpression,
       'arguments': jsArguments,
       'executionContextId': await contextId,
       'returnByValue': returnByValue,
     });
-    handleErrorIfPresent(result, evalContents: evalExpression);
-    return RemoteObject(result.result['result'] as Map<String, Object>);
+    final result =
+        getResultOrHandleError(response, evalContents: evalExpression);
+    return RemoteObject(result);
   }
 
   /// Invoke the function named [selector] on the object identified by
@@ -303,26 +304,28 @@ class AppInspector implements AppInspectorInterface {
   Future<RemoteObject> jsEvaluate(String expression,
       {bool returnByValue = false, bool awaitPromise = false}) async {
     // TODO(alanknight): Support a version with arguments if needed.
-    WipResponse result;
-    result = await remoteDebugger.sendCommand('Runtime.evaluate', params: {
+    final response =
+        await remoteDebugger.sendCommand('Runtime.evaluate', params: {
       'expression': expression,
       'returnByValue': returnByValue,
       'awaitPromise': awaitPromise,
       'contextId': await contextId,
     });
-    handleErrorIfPresent(result, evalContents: expression, additionalDetails: {
-      'Dart expression': expression,
-    });
-    return RemoteObject(result.result['result'] as Map<String, dynamic>);
+    final result = getResultOrHandleError(response, evalContents: expression);
+    return RemoteObject(result);
   }
 
   /// Evaluate the JS function with source [jsFunction] in the context of
   /// [library] with [arguments].
   Future<RemoteObject> _evaluateInLibrary(
       Library library, String jsFunction, List<RemoteObject> arguments) async {
+    final libraryUri = library.uri;
+    if (libraryUri == null) {
+      throwInvalidParam('invoke', 'library uri is null');
+    }
     final findLibrary = '''
 (function() {
-  ${globalLoadStrategy.loadLibrarySnippet(library.uri)};
+  ${globalLoadStrategy.loadLibrarySnippet(libraryUri)};
   return library;
 })();
 ''';
@@ -339,25 +342,25 @@ class AppInspector implements AppInspectorInterface {
   }
 
   @override
-  Future<InstanceRef> instanceRefFor(Object value) =>
+  Future<InstanceRef?> instanceRefFor(Object value) =>
       _instanceHelper.instanceRefFor(value);
 
-  Future<Instance> instanceFor(Object value) =>
+  Future<Instance?> instanceFor(RemoteObject value) =>
       _instanceHelper.instanceFor(value);
 
   @override
-  Future<LibraryRef> libraryRefFor(String objectId) =>
+  Future<LibraryRef?> libraryRefFor(String objectId) =>
       _libraryHelper.libraryRefFor(objectId);
 
   @override
-  Future<Library> getLibrary(String objectId) async {
+  Future<Library?> getLibrary(String objectId) async {
     final libraryRef = await libraryRefFor(objectId);
     if (libraryRef == null) return null;
     return _libraryHelper.libraryFor(libraryRef);
   }
 
   @override
-  Future<Obj> getObject(String objectId, {int offset, int count}) async {
+  Future<Obj?> getObject(String objectId, {int? offset, int? count}) async {
     try {
       final library = await getLibrary(objectId);
       if (library != null) {
@@ -384,9 +387,13 @@ class AppInspector implements AppInspectorInterface {
         'are supported for getObject');
   }
 
-  Future<Script> _getScript(ScriptRef scriptRef) async {
+  Future<Script?> _getScript(ScriptRef scriptRef) async {
     final libraryId = _scriptIdToLibraryId[scriptRef.id];
-    final serverPath = DartUri(scriptRef.uri, _root).serverPath;
+    final scriptUri = scriptRef.uri;
+    final scriptId = scriptRef.id;
+    if (libraryId == null || scriptUri == null || scriptId == null) return null;
+
+    final serverPath = DartUri(scriptUri, _root).serverPath;
     final source = await _assetReader.dartSourceContents(serverPath);
     if (source == null) {
       throw RPCError('getObject', RPCError.kInvalidParams,
@@ -395,16 +402,17 @@ class AppInspector implements AppInspectorInterface {
     return Script(
         uri: scriptRef.uri,
         library: await libraryRefFor(libraryId),
-        id: scriptRef.id)
+        id: scriptId)
       ..tokenPosTable = await _locations.tokenPosTableFor(serverPath)
       ..source = source;
   }
 
   @override
-  Future<MemoryUsage> getMemoryUsage() async {
+  Future<MemoryUsage?> getMemoryUsage() async {
     final response = await remoteDebugger.sendCommand('Runtime.getHeapUsage');
-
-    final jsUsage = HeapUsage(response.result);
+    final result = response.result;
+    if (result == null) return null;
+    final jsUsage = HeapUsage(result);
     return MemoryUsage.parse({
       'heapUsage': jsUsage.usedSize,
       'heapCapacity': jsUsage.totalSize,
@@ -414,7 +422,7 @@ class AppInspector implements AppInspectorInterface {
 
   /// Returns the [ScriptRef] for the provided Dart server path [uri].
   @override
-  Future<ScriptRef> scriptRefFor(String uri) async {
+  Future<ScriptRef?> scriptRefFor(String uri) async {
     await _populateScriptCaches();
     return _serverPathToScriptRef[uri];
   }
@@ -423,7 +431,7 @@ class AppInspector implements AppInspectorInterface {
   @override
   Future<List<ScriptRef>> scriptRefsForLibrary(String libraryId) async {
     await _populateScriptCaches();
-    return _libraryIdToScriptRefs[libraryId];
+    return _libraryIdToScriptRefs[libraryId] ?? [];
   }
 
   /// Return the VM SourceReport for the given parameters.
@@ -432,12 +440,12 @@ class AppInspector implements AppInspectorInterface {
   @override
   Future<SourceReport> getSourceReport(
     List<String> reports, {
-    String scriptId,
-    int tokenPos,
-    int endTokenPos,
-    bool forceCompile,
-    bool reportLines,
-    List<String> libraryFilters,
+    String? scriptId,
+    int? tokenPos,
+    int? endTokenPos,
+    bool? forceCompile,
+    bool? reportLines,
+    List<String>? libraryFilters,
   }) {
     if (reports.contains(SourceReportKind.kCoverage)) {
       throwInvalidParam('getSourceReport',
@@ -457,15 +465,19 @@ class AppInspector implements AppInspectorInterface {
     return _getPossibleBreakpoints(scriptId);
   }
 
-  Future<SourceReport> _getPossibleBreakpoints(String scriptId) async {
+  Future<SourceReport> _getPossibleBreakpoints(String? scriptId) async {
     // TODO(devoncarew): Consider adding some caching for this method.
 
     final scriptRef = scriptWithId(scriptId);
     if (scriptRef == null) {
-      throwInvalidParam('getSourceReport', 'scriptId not found: $scriptId');
+      throwInvalidParam('getSourceReport', 'scriptRef not found for $scriptId');
+    }
+    final scriptUri = scriptRef.uri;
+    if (scriptUri == null) {
+      throwInvalidParam('getSourceReport', 'scriptUri not found for $scriptId');
     }
 
-    final dartUri = DartUri(scriptRef.uri, _root);
+    final dartUri = DartUri(scriptUri, _root);
     final mappedLocations =
         await _locations.locationsForDart(dartUri.serverPath);
     // Unlike the Dart VM, the token positions match exactly to the possible
@@ -505,7 +517,9 @@ class AppInspector implements AppInspectorInterface {
   /// Returns the list of scripts refs cached.
   Future<List<ScriptRef>> _populateScriptCaches() async {
     return _scriptCacheMemoizer.runOnce(() async {
-      final libraryUris = [for (var library in isolate.libraries) library.uri];
+      final libraryUris = [
+        for (var library in isolate.libraries ?? []) library.uri
+      ];
       final scripts = await globalLoadStrategy
           .metadataProviderFor(appConnection.request.entrypointPath)
           .scripts;
@@ -517,16 +531,24 @@ class AppInspector implements AppInspectorInterface {
         final parts = scripts[uri];
         final scriptRefs = [
           ScriptRef(uri: uri, id: createId()),
-          for (var part in parts) ScriptRef(uri: part, id: createId())
+          for (var part in parts ?? []) ScriptRef(uri: part, id: createId())
         ];
         final libraryRef = await _libraryHelper.libraryRefFor(uri);
-        _libraryIdToScriptRefs.putIfAbsent(libraryRef.id, () => <ScriptRef>[]);
-        for (var scriptRef in scriptRefs) {
-          _scriptRefsById[scriptRef.id] = scriptRef;
-          _scriptIdToLibraryId[scriptRef.id] = libraryRef.id;
-          _serverPathToScriptRef[DartUri(scriptRef.uri, _root).serverPath] =
-              scriptRef;
-          _libraryIdToScriptRefs[libraryRef.id].add(scriptRef);
+        final libraryId = libraryRef?.id;
+        if (libraryId != null) {
+          final libraryIdToScriptRefs = _libraryIdToScriptRefs.putIfAbsent(
+              libraryId, () => <ScriptRef>[]);
+          for (var scriptRef in scriptRefs) {
+            final scriptId = scriptRef.id;
+            final scriptUri = scriptRef.uri;
+            if (scriptId != null && scriptUri != null) {
+              _scriptRefsById[scriptId] = scriptRef;
+              _scriptIdToLibraryId[scriptId] = libraryId;
+              _serverPathToScriptRef[DartUri(scriptUri, _root).serverPath] =
+                  scriptRef;
+              libraryIdToScriptRefs.add(scriptRef);
+            }
+          }
         }
       }
       return _scriptRefsById.values.toList();
@@ -535,7 +557,8 @@ class AppInspector implements AppInspectorInterface {
 
   /// Look up the script by id in an isolate.
   @override
-  ScriptRef scriptWithId(String scriptId) => _scriptRefsById[scriptId];
+  ScriptRef? scriptWithId(String? scriptId) =>
+      scriptId == null ? null : _scriptRefsById[scriptId];
 
   /// Runs an eval on the page to compute all existing registered extensions.
   Future<List<String>> _getExtensionRpcs() async {
@@ -548,11 +571,10 @@ class AppInspector implements AppInspectorInterface {
       'contextId': await contextId,
     };
     try {
-      final extensionsResult =
+      final response =
           await remoteDebugger.sendCommand('Runtime.evaluate', params: params);
-      handleErrorIfPresent(extensionsResult, evalContents: expression);
-      extensionRpcs.addAll(
-          List.from(extensionsResult.result['result']['value'] as List));
+      final result = getResultOrHandleError(response, evalContents: expression);
+      extensionRpcs.addAll(List.from(result['value'] as List? ?? []));
     } catch (e, s) {
       _logger.severe(
           'Error calling Runtime.evaluate with params $params', e, s);
@@ -571,7 +593,7 @@ class AppInspector implements AppInspectorInterface {
     } catch (_) {
       return description;
     }
-    final mappedStack = mapperResult?.value?.toString();
+    final mappedStack = mapperResult.value?.toString();
     if (mappedStack == null || mappedStack.isEmpty) {
       return description;
     }
diff --git a/dwds/lib/src/debugging/instance.dart b/dwds/lib/src/debugging/instance.dart
index f43a8214e..03851d933 100644
--- a/dwds/lib/src/debugging/instance.dart
+++ b/dwds/lib/src/debugging/instance.dart
@@ -275,6 +275,9 @@ class InstanceHelper extends Domain {
       int? count) async {
     final objectId = remoteObject.objectId;
     if (objectId == null) return null;
+
+    /// TODO(annagrin): split into cases to make the logic clear.
+    /// TODO(annagrin): make sure we use offset correctly.
     final numberOfProperties = _lengthOf(properties) ?? 0;
     final length = (offset == null && count == null)
         ? numberOfProperties
diff --git a/dwds/lib/src/services/chrome_proxy_service.dart b/dwds/lib/src/services/chrome_proxy_service.dart
index 7142bcba6..81973a4ff 100644
--- a/dwds/lib/src/services/chrome_proxy_service.dart
+++ b/dwds/lib/src/services/chrome_proxy_service.dart
@@ -364,10 +364,10 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
       'awaitPromise': true,
       'contextId': await executionContext.id,
     });
+    final result = await _inspector.jsEvaluate(expression, awaitPromise: true);
     handleErrorIfPresent(response, evalContents: expression);
     final decodedResponse =
-        jsonDecode(response.result['result']['value'] as String)
-            as Map<String, dynamic>;
+        jsonDecode(result.value as String) as Map<String, dynamic>;
     if (decodedResponse.containsKey('code') &&
         decodedResponse.containsKey('message') &&
         decodedResponse.containsKey('data')) {
diff --git a/dwds/lib/src/utilities/domain.dart b/dwds/lib/src/utilities/domain.dart
index b3d636a34..9829af561 100644
--- a/dwds/lib/src/utilities/domain.dart
+++ b/dwds/lib/src/utilities/domain.dart
@@ -20,7 +20,7 @@ abstract class AppInspectorInterface {
   IsolateRef get isolateRef;
 
   /// Returns the ID for the execution context or null if not found.
-  Future<int> get contextId;
+  Future<int?> get contextId;
 
   /// Calls Chrome's Runtime.callFunctionOn method.
   ///
@@ -34,7 +34,7 @@ abstract class AppInspectorInterface {
   Future<ScriptRef?> scriptRefFor(String uri);
 
   /// Look up the script by id in an isolate.
-  ScriptRef? scriptWithId(String scriptId);
+  ScriptRef? scriptWithId(String? scriptId);
 
   /// Returns the [LibraryRef] for the provided Dart [objectId].
   Future<LibraryRef?> libraryRefFor(String objectId);
@@ -76,7 +76,7 @@ abstract class AppInspectorInterface {
       {bool returnByValue = false, bool awaitPromise = false});
 
   /// Lookup an `object` from some isolate by its [objectId].
-  Future<Obj> getObject(String objectId, {int offset, int count});
+  Future<Obj?> getObject(String objectId, {int offset, int count});
 
   /// All the scripts in the isolate.
   Future<ScriptList> getScripts();
@@ -95,7 +95,7 @@ abstract class AppInspectorInterface {
   });
 
   /// Lookup current isolate's memory usage statistics.
-  Future<MemoryUsage> getMemoryUsage();
+  Future<MemoryUsage?> getMemoryUsage();
 }
 
 /// A common superclass to allow implementations of different parts of the
@@ -108,6 +108,6 @@ abstract class Domain {
   late AppInspectorInterface inspector;
 }
 
-void throwInvalidParam(String method, String message) {
+Never throwInvalidParam(String method, String message) {
   throw RPCError(method, RPCError.kInvalidParams, message);
 }
diff --git a/dwds/lib/src/utilities/shared.dart b/dwds/lib/src/utilities/shared.dart
index 5e021d4f5..fc82c397a 100644
--- a/dwds/lib/src/utilities/shared.dart
+++ b/dwds/lib/src/utilities/shared.dart
@@ -89,16 +89,44 @@ void serveHttpRequests(Stream<HttpRequest> requests, Handler handler,
 
 /// Throws an [wip.ExceptionDetails] object if `exceptionDetails` is present on the
 /// result.
-void handleErrorIfPresent(wip.WipResponse? response,
-    {String? evalContents, Object? additionalDetails}) {
-  if (response == null || response.result == null) return;
-  if (response.result!.containsKey('exceptionDetails')) {
-    final exceptionDetails =
-        response.result!['exceptionDetails']! as Map<String, dynamic>;
-    final stackTrace = wip.StackTrace(exceptionDetails['stackTrace']);
-    throw ChromeDebugException(exceptionDetails,
-        evalContents: evalContents,
-        additionalDetails: additionalDetails,
-        stackTrace: stackTrace);
+void handleErrorIfPresent(wip.WipResponse? response, {String? evalContents}) {
+  final result = response?.result;
+  if (result == null) return;
+  final exceptionDetails = result['exceptionDetails'] as Map<String, dynamic>?;
+  if (exceptionDetails != null) {
+    throwChromeDebugException(
+      exceptionDetails,
+      evalContents: evalContents,
+    );
+  }
+}
+
+/// Returns result contained in the response.
+/// Throws an [wip.ExceptionDetails] object if `exceptionDetails` is present on the
+/// result or the result is null.
+Map<String, dynamic> getResultOrHandleError(wip.WipResponse? response,
+    {String? evalContents}) {
+  final result = response?.result;
+  final ret = result?['result'];
+  final exceptionDetails = result?['exceptionDetails'] as Map<String, dynamic>?;
+  if (result == null || ret == null || exceptionDetails != null) {
+    throwChromeDebugException(
+      exceptionDetails,
+      evalContents: evalContents,
+    );
   }
+  return ret;
+}
+
+Never throwChromeDebugException(Map<String, dynamic>? exceptionDetails,
+    {String? evalContents, Object? additionalDetails}) {
+  final stackTraceDetails = exceptionDetails?['stackTrace'];
+  final stackTrace =
+      stackTraceDetails == null ? null : wip.StackTrace(stackTraceDetails);
+  throw ChromeDebugException(
+    exceptionDetails ?? {'text': 'null result from Chrome Devtools'},
+    evalContents: evalContents,
+    additionalDetails: additionalDetails,
+    stackTrace: stackTrace,
+  );
 }
diff --git a/dwds/test/inspector_test.dart b/dwds/test/inspector_test.dart
index e011addbc..24abc75f3 100644
--- a/dwds/test/inspector_test.dart
+++ b/dwds/test/inspector_test.dart
@@ -5,6 +5,7 @@
 // @dart = 2.9
 
 @TestOn('vm')
+import 'package:dwds/dwds.dart';
 import 'package:dwds/src/connections/debug_connection.dart';
 import 'package:dwds/src/debugging/debugger.dart';
 import 'package:dwds/src/debugging/inspector.dart';
@@ -48,6 +49,43 @@ void main() {
   Future<RemoteObject> libraryPrivate() =>
       inspector.jsEvaluate(libraryVariableExpression('_libraryPrivate'));
 
+  group('jsEvaluate', () {
+    test('no error', () async {
+      await expectLater(inspector.jsEvaluate('42'),
+          completion(isA<RemoteObject>().having((e) => e.value, 'value', 42)));
+    });
+
+    test('syntax error', () async {
+      await expectLater(
+          () => inspector.jsEvaluate('<'),
+          throwsA(isA<ChromeDebugException>()
+              .having((e) => e.text, 'text', 'Uncaught')
+              .having((e) => e.exception.description, 'description',
+                  contains('SyntaxError'))
+              .having((e) => e.stackTrace, 'stackTrace', isNull)
+              .having((e) => e.evalContents, 'evalContents', '<')));
+    });
+
+    test('evaluation error', () async {
+      await expectLater(
+          () => inspector.jsEvaluate('''
+        (function() {
+          let foo = (function() {
+            console.log(boo);
+          });
+          foo();
+        })();
+        '''),
+          throwsA(isA<ChromeDebugException>()
+              .having((e) => e.text, 'text', 'Uncaught')
+              .having((e) => e.exception.description, 'description',
+                  contains('ReferenceError'))
+              .having((e) => e.stackTrace.printFrames()[0], 'stackTrace',
+                  contains('foo()'))
+              .having((e) => e.evalContents, 'evalContents', contains('foo'))));
+    });
+  });
+
   test('send toString', () async {
     final remoteObject = await libraryPublicFinal();
     final toString =

From 5a74b9b0c33826fdcdf2b43203d127daa45c2053 Mon Sep 17 00:00:00 2001
From: Anna Gringauze <annagrin@google.com>
Date: Wed, 13 Jul 2022 14:59:30 -0700
Subject: [PATCH 2/7] Migrate expression evaluator to null safety

---
 dwds/lib/src/debugging/debugger.dart          | 10 +--
 dwds/lib/src/debugging/inspector.dart         |  5 ++
 .../src/services/chrome_proxy_service.dart    | 10 ---
 .../src/services/expression_evaluator.dart    | 68 ++++++++++---------
 dwds/lib/src/utilities/domain.dart            |  3 +
 dwds/test/debugger_test.dart                  |  4 +-
 dwds/test/fixtures/fakes.dart                 |  8 ++-
 7 files changed, 57 insertions(+), 51 deletions(-)

diff --git a/dwds/lib/src/debugging/debugger.dart b/dwds/lib/src/debugging/debugger.dart
index 9ab72eb76..1978f0d81 100644
--- a/dwds/lib/src/debugging/debugger.dart
+++ b/dwds/lib/src/debugging/debugger.dart
@@ -331,10 +331,6 @@ class Debugger extends Domain {
     }
   }
 
-  /// Returns Chrome script uri for Chrome script ID.
-  String? urlForScriptId(String scriptId) =>
-      _remoteDebugger.scripts[scriptId]?.url;
-
   /// Returns source [Location] for the paused event.
   ///
   /// If we do not have [Location] data for the embedded JS location, null is
@@ -349,7 +345,7 @@ class Debugger extends Domain {
     if (scriptId == null || line == null) return null;
 
     final column = location['columnNumber'] as int?;
-    final url = urlForScriptId(scriptId);
+    final url = inspector.urlForScriptId(scriptId);
     if (url == null) return null;
 
     return _locations.locationForJs(url, line, column);
@@ -499,7 +495,7 @@ class Debugger extends Domain {
     final line = location.lineNumber;
     final column = location.columnNumber;
 
-    final url = urlForScriptId(location.scriptId);
+    final url = inspector.urlForScriptId(location.scriptId);
     if (url == null) {
       logger.severe('Failed to create dart frame for ${frame.functionName}: '
           'cannot find url for script ${location.scriptId}');
@@ -602,7 +598,7 @@ class Debugger extends Domain {
               'cannot find script id for event $e');
           throw StateError('Stepping failed on event $e');
         }
-        final url = urlForScriptId(scriptId);
+        final url = inspector.urlForScriptId(scriptId);
         if (url == null) {
           logger.severe('Stepping failed: '
               'cannot find url for script $scriptId');
diff --git a/dwds/lib/src/debugging/inspector.dart b/dwds/lib/src/debugging/inspector.dart
index 080dfb1be..920647d77 100644
--- a/dwds/lib/src/debugging/inspector.dart
+++ b/dwds/lib/src/debugging/inspector.dart
@@ -560,6 +560,11 @@ class AppInspector implements AppInspectorInterface {
   ScriptRef? scriptWithId(String? scriptId) =>
       scriptId == null ? null : _scriptRefsById[scriptId];
 
+  /// Returns Chrome script uri for Chrome script ID.
+  @override
+  String? urlForScriptId(String scriptId) =>
+      remoteDebugger.scripts[scriptId]?.url;
+
   /// Runs an eval on the page to compute all existing registered extensions.
   Future<List<String>> _getExtensionRpcs() async {
     final expression =
diff --git a/dwds/lib/src/services/chrome_proxy_service.dart b/dwds/lib/src/services/chrome_proxy_service.dart
index 81973a4ff..16724c45f 100644
--- a/dwds/lib/src/services/chrome_proxy_service.dart
+++ b/dwds/lib/src/services/chrome_proxy_service.dart
@@ -458,16 +458,6 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
         await isCompilerInitialized;
         _checkIsolate('evaluateInFrame', isolateId);
 
-        if (scope != null) {
-          // TODO(annagrin): Implement scope support.
-          // Issue: https://github.com/dart-lang/webdev/issues/1344
-          throw RPCError(
-              'evaluateInFrame',
-              RPCError.kInvalidRequest,
-              'Expression evaluation with scope is not supported '
-                  'for this configuration.');
-        }
-
         return await _getEvaluationResult(
             () => _expressionEvaluator.evaluateExpressionInFrame(
                 isolateId, frameIndex, expression, scope),
diff --git a/dwds/lib/src/services/expression_evaluator.dart b/dwds/lib/src/services/expression_evaluator.dart
index 32e9f4ed7..5f15e7616 100644
--- a/dwds/lib/src/services/expression_evaluator.dart
+++ b/dwds/lib/src/services/expression_evaluator.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:dwds/src/utilities/domain.dart';
 import 'package:logging/logging.dart';
 import 'package:webkit_inspection_protocol/webkit_inspection_protocol.dart';
@@ -78,19 +76,18 @@ class ExpressionEvaluator {
     String isolateId,
     String libraryUri,
     String expression,
-    Map<String, String> scope,
+    Map<String, String>? scope,
   ) async {
     scope ??= {};
-    if (_compiler == null) {
-      return _createError(ErrorKind.internal,
-          'ExpressionEvaluator needs an ExpressionCompiler');
-    }
 
-    if (expression == null || expression.isEmpty) {
+    if (expression.isEmpty) {
       return _createError(ErrorKind.invalidInput, expression);
     }
 
     final module = await _modules.moduleForlibrary(libraryUri);
+    if (module == null) {
+      return _createError(ErrorKind.internal, 'no module for $libraryUri');
+    }
 
     // Wrap the expression in a lambda so we can call it as a function.
     expression = _createDartLambda(expression, scope.keys);
@@ -132,13 +129,17 @@ class ExpressionEvaluator {
   /// [frameIndex] JavaScript frame to evaluate the expression in.
   /// [expression] dart expression to evaluate.
   Future<RemoteObject> evaluateExpressionInFrame(String isolateId,
-      int frameIndex, String expression, Map<String, String> scope) async {
-    if (_compiler == null) {
-      return _createError(ErrorKind.internal,
-          'ExpressionEvaluator needs an ExpressionCompiler');
+      int frameIndex, String expression, Map<String, String>? scope) async {
+    if (scope != null) {
+      // TODO(annagrin): Implement scope support.
+      // Issue: https://github.com/dart-lang/webdev/issues/1344
+      return _createError(
+          ErrorKind.internal,
+          'Using scope for expression evaluation in frame '
+          'is not supported.');
     }
 
-    if (expression == null || expression.isEmpty) {
+    if (expression.isEmpty) {
       return _createError(ErrorKind.invalidInput, expression);
     }
 
@@ -158,7 +159,11 @@ class ExpressionEvaluator {
     final jsScope = await _collectLocalJsScope(jsFrame);
 
     // Find corresponding dart location and scope.
-    final url = _urlForScriptId(jsScriptId);
+    final url = _inspector.urlForScriptId(jsScriptId);
+    if (url == null) {
+      return _createError(
+          ErrorKind.internal, 'Cannot find url for JS script: $jsScriptId');
+    }
     final locationMap = await _locations.locationForJs(url, jsLine, jsColumn);
     if (locationMap == null) {
       return _createError(
@@ -171,13 +176,16 @@ class ExpressionEvaluator {
     }
 
     final dartLocation = locationMap.dartLocation;
-    final libraryUri =
-        await _modules.libraryForSource(dartLocation.uri.serverPath);
+    final dartSourcePath = dartLocation.uri.serverPath;
+    final libraryUri = await _modules.libraryForSource(dartSourcePath);
 
-    final currentModule =
-        await _modules.moduleForSource(dartLocation.uri.serverPath);
+    final module = await _modules.moduleForlibrary(dartSourcePath);
+    if (module == null) {
+      return _createError(
+          ErrorKind.internal, 'no module for $libraryUri ($dartSourcePath)');
+    }
 
-    _logger.finest('Evaluating "$expression" at $currentModule, '
+    _logger.finest('Evaluating "$expression" at $module, '
         '$libraryUri:${dartLocation.line}:${dartLocation.column}');
 
     // Compile expression using an expression compiler, such as
@@ -189,7 +197,7 @@ class ExpressionEvaluator {
         dartLocation.column,
         {},
         jsScope,
-        currentModule,
+        module,
         expression);
 
     final isError = compilationResult.isError;
@@ -267,7 +275,9 @@ class ExpressionEvaluator {
       for (var p in variables) {
         final name = p.name;
         final value = p.value;
-        if (!_isUndefined(value)) {
+        // TODO: null values represent variables optimized by v8.
+        // Show that to the user.
+        if (name != null && value != null && !_isUndefined(value)) {
           jsScope[name] = name;
         }
       }
@@ -276,17 +286,17 @@ class ExpressionEvaluator {
     // skip library and main scope
     final scopeChain = filterScopes(frame).reversed;
     for (var scope in scopeChain) {
-      final scopeProperties =
-          await _debugger.getProperties(scope.object.objectId);
-
-      collectVariables(scope.scope, scopeProperties);
+      final objectId = scope.object.objectId;
+      if (objectId != null) {
+        final scopeProperties = await _debugger.getProperties(objectId);
+        collectVariables(scope.scope, scopeProperties);
+      }
     }
 
     return jsScope;
   }
 
-  bool _isUndefined(RemoteObject value) =>
-      value == null || value.type == 'undefined';
+  bool _isUndefined(RemoteObject value) => value.type == 'undefined';
 
   /// Strip try/catch incorrectly added by the expression compiler.
   /// TODO: remove adding try/catch block in expression compiler.
@@ -346,8 +356,4 @@ class ExpressionEvaluator {
 
   String _createDartLambda(String expression, Iterable<String> params) =>
       '(${params.join(', ')}) => $expression';
-
-  /// Returns Chrome script uri for Chrome script ID.
-  String _urlForScriptId(String scriptId) =>
-      _inspector.remoteDebugger.scripts[scriptId]?.url;
 }
diff --git a/dwds/lib/src/utilities/domain.dart b/dwds/lib/src/utilities/domain.dart
index 9829af561..48304e2a4 100644
--- a/dwds/lib/src/utilities/domain.dart
+++ b/dwds/lib/src/utilities/domain.dart
@@ -36,6 +36,9 @@ abstract class AppInspectorInterface {
   /// Look up the script by id in an isolate.
   ScriptRef? scriptWithId(String? scriptId);
 
+  /// Returns Chrome script uri for Chrome script ID.
+  String? urlForScriptId(String scriptId);
+
   /// Returns the [LibraryRef] for the provided Dart [objectId].
   Future<LibraryRef?> libraryRefFor(String objectId);
 
diff --git a/dwds/test/debugger_test.dart b/dwds/test/debugger_test.dart
index d2e45ccb6..d9a4c9577 100644
--- a/dwds/test/debugger_test.dart
+++ b/dwds/test/debugger_test.dart
@@ -111,7 +111,7 @@ void main() async {
       skipLists,
       root,
     );
-    inspector = FakeInspector();
+    inspector = FakeInspector(webkitDebugger);
     debugger.updateInspector(inspector);
   });
 
@@ -182,7 +182,7 @@ void main() async {
     setUp(() {
       // We need to provide an Isolate so that the code doesn't bail out on a null
       // check before it has a chance to throw.
-      inspector = FakeInspector(fakeIsolate: simpleIsolate);
+      inspector = FakeInspector(webkitDebugger, fakeIsolate: simpleIsolate);
       debugger.updateInspector(inspector);
     });
 
diff --git a/dwds/test/fixtures/fakes.dart b/dwds/test/fixtures/fakes.dart
index 4e3904caf..40c9f0dd2 100644
--- a/dwds/test/fixtures/fakes.dart
+++ b/dwds/test/fixtures/fakes.dart
@@ -47,10 +47,13 @@ Isolate get simpleIsolate => Isolate(
     );
 
 class FakeInspector implements AppInspector {
-  FakeInspector({this.fakeIsolate});
+  FakeInspector(this.remoteDebugger, {this.fakeIsolate});
 
   Isolate fakeIsolate;
 
+  @override
+  RemoteDebugger remoteDebugger;
+
   final _instanceHelper = InstanceHelper(null, null);
 
   @override
@@ -80,6 +83,9 @@ class FakeInspector implements AppInspector {
   @override
   ScriptRef scriptWithId(String scriptId) => null;
 
+  @override
+  String urlForScriptId(String scriptId) => remoteDebugger.scripts[scriptId].url;
+
   @override
   Isolate get isolate => fakeIsolate;
 

From 7b66c3bef53592e589675160902671052260c0a1 Mon Sep 17 00:00:00 2001
From: Anna Gringauze <annagrin@google.com>
Date: Wed, 13 Jul 2022 15:05:22 -0700
Subject: [PATCH 3/7] format

---
 dwds/test/fixtures/fakes.dart | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/dwds/test/fixtures/fakes.dart b/dwds/test/fixtures/fakes.dart
index 40c9f0dd2..81067bfa8 100644
--- a/dwds/test/fixtures/fakes.dart
+++ b/dwds/test/fixtures/fakes.dart
@@ -84,7 +84,8 @@ class FakeInspector implements AppInspector {
   ScriptRef scriptWithId(String scriptId) => null;
 
   @override
-  String urlForScriptId(String scriptId) => remoteDebugger.scripts[scriptId].url;
+  String urlForScriptId(String scriptId) =>
+      remoteDebugger.scripts[scriptId].url;
 
   @override
   Isolate get isolate => fakeIsolate;

From f0074703db221132d33da9ba7b1a601a9a39dc0c Mon Sep 17 00:00:00 2001
From: Anna Gringauze <annagrin@google.com>
Date: Wed, 13 Jul 2022 16:49:54 -0700
Subject: [PATCH 4/7] Fix failing tests

---
 .../src/services/expression_evaluator.dart    |  6 +++++-
 dwds/test/evaluate_common.dart                | 21 ++++++++++++-------
 2 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/dwds/lib/src/services/expression_evaluator.dart b/dwds/lib/src/services/expression_evaluator.dart
index 5f15e7616..9b6ff6809 100644
--- a/dwds/lib/src/services/expression_evaluator.dart
+++ b/dwds/lib/src/services/expression_evaluator.dart
@@ -178,8 +178,12 @@ class ExpressionEvaluator {
     final dartLocation = locationMap.dartLocation;
     final dartSourcePath = dartLocation.uri.serverPath;
     final libraryUri = await _modules.libraryForSource(dartSourcePath);
+    if (libraryUri == null) {
+      return _createError(
+          ErrorKind.internal, 'no libraryUri for $dartSourcePath');
+    }
 
-    final module = await _modules.moduleForlibrary(dartSourcePath);
+    final module = await _modules.moduleForlibrary(libraryUri.toString());
     if (module == null) {
       return _createError(
           ErrorKind.internal, 'no module for $libraryUri ($dartSourcePath)');
diff --git a/dwds/test/evaluate_common.dart b/dwds/test/evaluate_common.dart
index 797ab475f..4c8eb17de 100644
--- a/dwds/test/evaluate_common.dart
+++ b/dwds/test/evaluate_common.dart
@@ -145,15 +145,20 @@ void testAll({
               isolate.id, event.topFrame.index, 'MainClass(0)');
 
           final param = object as InstanceRef;
+          final result = await setup.service.evaluateInFrame(
+            isolate.id,
+            event.topFrame.index,
+            't.toString()',
+            scope: {'t': param.id},
+          );
 
-          await expectLater(
-              () => setup.service.evaluateInFrame(
-                    isolate.id,
-                    event.topFrame.index,
-                    't.toString()',
-                    scope: {'t': param.id},
-                  ),
-              throwsRPCError);
+          expect(
+              result,
+              isA<ErrorRef>().having(
+                  (instance) => instance.message,
+                  'message',
+                  contains('Using scope for expression evaluation '
+                      'in frame is not supported')));
         });
       });
 

From 5acb6c743edb60b8d5db83d5a0bb6db46889a0a6 Mon Sep 17 00:00:00 2001
From: Anna Gringauze <annagrin@google.com>
Date: Thu, 14 Jul 2022 10:58:17 -0700
Subject: [PATCH 5/7] Migrate services/chrome_proxu_service.dart to null safety

---
 dwds/lib/src/debugging/inspector.dart         |  18 +-
 dwds/lib/src/events.dart                      |   3 +-
 .../src/services/chrome_proxy_service.dart    | 288 +++++++++---------
 .../src/services/expression_evaluator.dart    |   6 +-
 dwds/lib/src/utilities/domain.dart            |   2 +-
 5 files changed, 168 insertions(+), 149 deletions(-)

diff --git a/dwds/lib/src/debugging/inspector.dart b/dwds/lib/src/debugging/inspector.dart
index 920647d77..bb20135f9 100644
--- a/dwds/lib/src/debugging/inspector.dart
+++ b/dwds/lib/src/debugging/inspector.dart
@@ -396,8 +396,8 @@ class AppInspector implements AppInspectorInterface {
     final serverPath = DartUri(scriptUri, _root).serverPath;
     final source = await _assetReader.dartSourceContents(serverPath);
     if (source == null) {
-      throw RPCError('getObject', RPCError.kInvalidParams,
-          'Failed to load script at path: $serverPath');
+      throwInvalidParam('getObject',
+          'No source for script $scriptId (server path: $serverPath)');
     }
     return Script(
         uri: scriptRef.uri,
@@ -408,16 +408,24 @@ class AppInspector implements AppInspectorInterface {
   }
 
   @override
-  Future<MemoryUsage?> getMemoryUsage() async {
+  Future<MemoryUsage> getMemoryUsage() async {
     final response = await remoteDebugger.sendCommand('Runtime.getHeapUsage');
     final result = response.result;
-    if (result == null) return null;
+    if (result == null) {
+      throw RPCError('getMemoryUsage', RPCError.kInternalError,
+          'Null result from chrome Devtools.');
+    }
     final jsUsage = HeapUsage(result);
-    return MemoryUsage.parse({
+    final usage = MemoryUsage.parse({
       'heapUsage': jsUsage.usedSize,
       'heapCapacity': jsUsage.totalSize,
       'externalUsage': 0,
     });
+    if (usage == null) {
+      throw RPCError('getMemoryUsage', RPCError.kInternalError,
+          'Failed to parse memory usage result.');
+    }
+    return usage;
   }
 
   /// Returns the [ScriptRef] for the provided Dart server path [uri].
diff --git a/dwds/lib/src/events.dart b/dwds/lib/src/events.dart
index c34d1b8f6..9cdb13bbe 100644
--- a/dwds/lib/src/events.dart
+++ b/dwds/lib/src/events.dart
@@ -85,7 +85,8 @@ class DwdsEvent {
 
   DwdsEvent.getVM() : this(DwdsEventKind.getVM, {});
 
-  DwdsEvent.resume(String step) : this(DwdsEventKind.resume, {'step': step});
+  DwdsEvent.resume(String? step)
+      : this(DwdsEventKind.resume, {if (step != null) 'step': step});
 
   DwdsEvent.getSourceReport() : this(DwdsEventKind.getSourceReport, {});
 
diff --git a/dwds/lib/src/services/chrome_proxy_service.dart b/dwds/lib/src/services/chrome_proxy_service.dart
index 16724c45f..8f95a8184 100644
--- a/dwds/lib/src/services/chrome_proxy_service.dart
+++ b/dwds/lib/src/services/chrome_proxy_service.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
@@ -27,7 +25,6 @@ import '../debugging/skip_list.dart';
 import '../events.dart';
 import '../loaders/strategy.dart';
 import '../readers/asset_reader.dart';
-import '../services/chrome_debug_exception.dart';
 import '../services/expression_compiler.dart';
 import '../utilities/dart_uri.dart';
 import '../utilities/sdk_configuration.dart';
@@ -71,18 +68,32 @@ class ChromeProxyService implements VmServiceInterface {
   Future<Debugger> get debuggerFuture => _debuggerCompleter.future;
   final _debuggerCompleter = Completer<Debugger>();
 
-  AppInspector get inspector => _inspector;
-  AppInspector _inspector;
+  /// Provides variable inspection functionality.
+  AppInspector get inspector {
+    if (_inspector == null) {
+      throw StateError('No running isolate (inspector is not set).');
+    }
+    return _inspector!;
+  }
+
+  AppInspector? _inspector;
 
-  StreamSubscription<ConsoleAPIEvent> _consoleSubscription;
+  /// Determines if there an isolate running currently.
+  ///
+  /// [_inspector] is `null` iff the isolate is not running,
+  /// for example, before the first isolate starts or during
+  /// a hot restart.
+  bool get _isIsolateRunning => _inspector != null;
+
+  StreamSubscription<ConsoleAPIEvent>? _consoleSubscription;
 
   final _disabledBreakpoints = <Breakpoint>{};
   final _previousBreakpoints = <Breakpoint>{};
 
   final _logger = Logger('ChromeProxyService');
 
-  final ExpressionCompiler _compiler;
-  ExpressionEvaluator _expressionEvaluator;
+  final ExpressionCompiler? _compiler;
+  ExpressionEvaluator? _expressionEvaluator;
 
   final SdkConfigurationProvider _sdkConfigurationProvider;
 
@@ -172,13 +183,14 @@ class ChromeProxyService implements VmServiceInterface {
     _logger.info('Initializing expression compiler for $entrypoint '
         'with sound null safety: $soundNullSafety');
 
-    if (_compiler != null) {
-      await _compiler?.initialize(
+    final compiler = _compiler;
+    if (compiler != null) {
+      await compiler.initialize(
           moduleFormat: moduleFormat, soundNullSafety: soundNullSafety);
       final dependencies =
           await globalLoadStrategy.moduleInfoForEntrypoint(entrypoint);
       await captureElapsedTime(() async {
-        final result = await _compiler.updateDependencies(dependencies);
+        final result = await compiler.updateDependencies(dependencies);
         // Expression evaluation is ready after dependencies are updated.
         if (!_compilerCompleter.isCompleted) _compilerCompleter.complete();
         return result;
@@ -192,7 +204,8 @@ class ChromeProxyService implements VmServiceInterface {
   /// with [destroyIsolate] and recreated with this method there is a hot
   /// restart or full page refresh.
   Future<void> createIsolate(AppConnection appConnection) async {
-    if (_inspector?.isolate != null) {
+    // Inspector is null if the previous isolate is destroyed.
+    if (_isIsolateRunning) {
       throw UnsupportedError(
           'Cannot create multiple isolates for the same app');
     }
@@ -219,17 +232,18 @@ class ChromeProxyService implements VmServiceInterface {
       executionContext,
       sdkConfiguration,
     );
-    debugger.updateInspector(_inspector);
+    debugger.updateInspector(inspector);
 
-    _expressionEvaluator = _compiler == null
+    final compiler = _compiler;
+    _expressionEvaluator = compiler == null
         ? null
         : ExpressionEvaluator(
             entrypoint,
-            _inspector,
+            inspector,
             debugger,
             _locations,
             _modules,
-            _compiler,
+            compiler,
           );
 
     await debugger.reestablishBreakpoints(
@@ -240,13 +254,13 @@ class ChromeProxyService implements VmServiceInterface {
       await debugger.resumeFromStart();
     }));
 
-    final isolateRef = _inspector.isolateRef;
+    final isolateRef = inspector.isolateRef;
     final timestamp = DateTime.now().millisecondsSinceEpoch;
 
     // Listen for `registerExtension` and `postEvent` calls.
     _setUpChromeConsoleListeners(isolateRef);
 
-    _vm.isolates.add(isolateRef);
+    _vm.isolates?.add(isolateRef);
 
     _streamNotify(
         'Isolate',
@@ -264,7 +278,7 @@ class ChromeProxyService implements VmServiceInterface {
     // TODO: We shouldn't need to fire these events since they exist on the
     // isolate, but devtools doesn't recognize extensions after a page refresh
     // otherwise.
-    for (var extensionRpc in _inspector.isolate.extensionRPCs) {
+    for (var extensionRpc in inspector.isolate.extensionRPCs ?? []) {
       _streamNotify(
           'Isolate',
           Event(
@@ -282,8 +296,10 @@ class ChromeProxyService implements VmServiceInterface {
   ///
   /// Clears out the [_inspector] and all related cached information.
   void destroyIsolate() {
-    final isolate = _inspector?.isolate;
-    if (isolate == null) return;
+    if (!_isIsolateRunning) return;
+    final isolate = inspector.isolate;
+    final isolateRef = inspector.isolateRef;
+
     _initializedCompleter = Completer<void>();
     _compilerCompleter = Completer<void>();
     _streamNotify(
@@ -291,28 +307,29 @@ class ChromeProxyService implements VmServiceInterface {
         Event(
             kind: EventKind.kIsolateExit,
             timestamp: DateTime.now().millisecondsSinceEpoch,
-            isolate: _inspector.isolateRef));
-    _vm.isolates.removeWhere((ref) => ref.id == isolate.id);
+            isolate: isolateRef));
+    _vm.isolates?.removeWhere((ref) => ref.id == isolate.id);
     _inspector = null;
     _previousBreakpoints.clear();
-    _previousBreakpoints.addAll(isolate.breakpoints);
+    _previousBreakpoints.addAll(isolate.breakpoints ?? []);
     _consoleSubscription?.cancel();
     _consoleSubscription = null;
   }
 
   Future<void> disableBreakpoints() async {
     _disabledBreakpoints.clear();
-    final isolate = _inspector?.isolate;
-    if (isolate == null) return;
-    _disabledBreakpoints.addAll(isolate.breakpoints);
-    for (var breakpoint in isolate.breakpoints.toList()) {
+    if (!_isIsolateRunning) return;
+    final isolate = inspector.isolate;
+
+    _disabledBreakpoints.addAll(isolate.breakpoints ?? []);
+    for (var breakpoint in isolate.breakpoints?.toList() ?? []) {
       await (await debuggerFuture).removeBreakpoint(breakpoint.id);
     }
   }
 
   @override
   Future<Breakpoint> addBreakpoint(String isolateId, String scriptId, int line,
-      {int column}) async {
+      {int? column}) async {
     await isInitialized;
     _checkIsolate('addBreakpoint', isolateId);
     return (await debuggerFuture).addBreakpoint(scriptId, line, column: column);
@@ -326,30 +343,38 @@ class ChromeProxyService implements VmServiceInterface {
   @override
   Future<Breakpoint> addBreakpointWithScriptUri(
       String isolateId, String scriptUri, int line,
-      {int column}) async {
+      {int? column}) async {
     await isInitialized;
     _checkIsolate('addBreakpointWithScriptUri', isolateId);
     if (Uri.parse(scriptUri).scheme == 'dart') {
-      _logger.finest('Cannot set breakpoint at $scriptUri:$line:$column: '
-          'breakpoints in dart SDK locations are not supported yet.');
       // TODO(annagrin): Support setting breakpoints in dart SDK locations.
       // Issue: https://github.com/dart-lang/webdev/issues/1584
       throw RPCError(
           'addBreakpoint',
           102,
           'The VM is unable to add a breakpoint '
-              'at the specified line or function');
+              'at the specified line or function: $scriptUri:$line:$column: '
+              'breakpoints in dart SDK locations are not supported yet.');
     }
     final dartUri = DartUri(scriptUri, root);
-    final ref = await _inspector.scriptRefFor(dartUri.serverPath);
-    return (await debuggerFuture).addBreakpoint(ref.id, line, column: column);
+    final scriptRef = await inspector.scriptRefFor(dartUri.serverPath);
+    final scriptId = scriptRef?.id;
+    if (scriptId == null) {
+      throw RPCError(
+          'addBreakpoint',
+          102,
+          'The VM is unable to add a breakpoint '
+              'at the specified line or function: $scriptUri:$line:$column: '
+              'cannot find script ID for ${dartUri.serverPath}');
+    }
+    return (await debuggerFuture).addBreakpoint(scriptId, line, column: column);
   }
 
   @override
   Future<Response> callServiceExtension(String method,
-      {String isolateId, Map args}) async {
+      {String? isolateId, Map? args}) async {
     await isInitialized;
-    isolateId ??= _inspector.isolate.id;
+    isolateId ??= _inspector?.isolate.id;
     _checkIsolate('callServiceExtension', isolateId);
     args ??= <String, String>{};
     final stringArgs = args.map((k, v) => MapEntry(
@@ -358,14 +383,7 @@ class ChromeProxyService implements VmServiceInterface {
 ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
     "$method", JSON.stringify(${jsonEncode(stringArgs)}));
 ''';
-    final response =
-        await remoteDebugger.sendCommand('Runtime.evaluate', params: {
-      'expression': expression,
-      'awaitPromise': true,
-      'contextId': await executionContext.id,
-    });
-    final result = await _inspector.jsEvaluate(expression, awaitPromise: true);
-    handleErrorIfPresent(response, evalContents: expression);
+    final result = await inspector.jsEvaluate(expression, awaitPromise: true);
     final decodedResponse =
         jsonDecode(result.value as String) as Map<String, dynamic>;
     if (decodedResponse.containsKey('code') &&
@@ -405,7 +423,7 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
           id: createId(),
         );
       }
-      return _inspector?.instanceRefFor(result);
+      return (await _instanceRef(result));
     } on RPCError catch (_) {
       rethrow;
     } catch (e, s) {
@@ -425,20 +443,21 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
     String isolateId,
     String targetId,
     String expression, {
-    Map<String, String> scope,
-    bool disableBreakpoints,
+    Map<String, String>? scope,
+    bool? disableBreakpoints,
   }) async {
     // TODO(798) - respect disableBreakpoints.
     return captureElapsedTime(() async {
       await isInitialized;
-      if (_expressionEvaluator != null) {
+      final evaluator = _expressionEvaluator;
+      if (evaluator != null) {
         await isCompilerInitialized;
         _checkIsolate('evaluate', isolateId);
 
-        final library = await _inspector?.getLibrary(targetId);
+        final library = await inspector.getLibrary(targetId);
         return await _getEvaluationResult(
-            () => _expressionEvaluator.evaluateExpression(
-                isolateId, library.uri, expression, scope),
+            () => evaluator.evaluateExpression(
+                isolateId, library?.uri, expression, scope),
             expression);
       }
       throw RPCError('evaluateInFrame', RPCError.kInvalidRequest,
@@ -449,17 +468,18 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   @override
   Future<Response> evaluateInFrame(
       String isolateId, int frameIndex, String expression,
-      {Map<String, String> scope, bool disableBreakpoints}) async {
+      {Map<String, String>? scope, bool? disableBreakpoints}) async {
     // TODO(798) - respect disableBreakpoints.
 
     return captureElapsedTime(() async {
       await isInitialized;
-      if (_expressionEvaluator != null) {
+      final evaluator = _expressionEvaluator;
+      if (evaluator != null) {
         await isCompilerInitialized;
         _checkIsolate('evaluateInFrame', isolateId);
 
         return await _getEvaluationResult(
-            () => _expressionEvaluator.evaluateExpressionInFrame(
+            () => evaluator.evaluateExpressionInFrame(
                 isolateId, frameIndex, expression, scope),
             expression);
       }
@@ -470,7 +490,7 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
 
   @override
   Future<AllocationProfile> getAllocationProfile(String isolateId,
-      {bool gc, bool reset}) {
+      {bool? gc, bool? reset}) {
     return _rpcNotSupportedFuture('getAllocationProfile');
   }
 
@@ -492,22 +512,12 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
     return _rpcNotSupportedFuture('getInstances');
   }
 
-  /// Sync version of [getIsolate] for internal use, also has stronger typing
-  /// than the public one which has to be dynamic.
-  Isolate _getIsolate(String isolateId) {
-    final isolate = _inspector?.isolate;
-    if (isolate?.id == isolateId) return isolate;
-    // TODO: Throw an RPC error here.
-    throw ArgumentError.value(isolateId, 'isolateId',
-        'Unrecognized isolate id. (Supported isolate: ${isolate?.id})');
-  }
-
   @override
   Future<Isolate> getIsolate(String isolateId) async {
     return captureElapsedTime(() async {
       await isInitialized;
       _checkIsolate('getIsolate', isolateId);
-      return _getIsolate(isolateId);
+      return inspector.isolate;
     }, (result) => DwdsEvent.getIsolate());
   }
 
@@ -515,15 +525,21 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   Future<MemoryUsage> getMemoryUsage(String isolateId) async {
     await isInitialized;
     _checkIsolate('getMemoryUsage', isolateId);
-    return _inspector.getMemoryUsage();
+    return inspector.getMemoryUsage();
   }
 
   @override
   Future<Obj> getObject(String isolateId, String objectId,
-      {int offset, int count}) async {
+      {int? offset, int? count}) async {
     await isInitialized;
     _checkIsolate('getObject', isolateId);
-    return _inspector?.getObject(objectId, offset: offset, count: count);
+    final result =
+        await inspector.getObject(objectId, offset: offset, count: count);
+    if (result == null) {
+      throw RPCError('getObject', RPCError.kInternalError,
+          'failed to find object for $objectId: null result');
+    }
+    return result;
   }
 
   @override
@@ -531,7 +547,7 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
     return await captureElapsedTime(() async {
       await isInitialized;
       _checkIsolate('getScripts', isolateId);
-      return _inspector?.getScripts();
+      return inspector.getScripts();
     }, (result) => DwdsEvent.getScripts());
   }
 
@@ -539,17 +555,17 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   Future<SourceReport> getSourceReport(
     String isolateId,
     List<String> reports, {
-    String scriptId,
-    int tokenPos,
-    int endTokenPos,
-    bool forceCompile,
-    bool reportLines,
-    List<String> libraryFilters,
+    String? scriptId,
+    int? tokenPos,
+    int? endTokenPos,
+    bool? forceCompile,
+    bool? reportLines,
+    List<String>? libraryFilters,
   }) async {
     return await captureElapsedTime(() async {
       await isInitialized;
       _checkIsolate('getSourceReport', isolateId);
-      return await _inspector?.getSourceReport(
+      return await inspector.getSourceReport(
         reports,
         scriptId: scriptId,
         tokenPos: tokenPos,
@@ -567,7 +583,7 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   ///
   /// The returned stack will contain up to [limit] frames if provided.
   @override
-  Future<Stack> getStack(String isolateId, {int limit}) async {
+  Future<Stack> getStack(String isolateId, {int? limit}) async {
     await isInitialized;
     _checkIsolate('getStack', isolateId);
     return (await debuggerFuture).getStack(limit: limit);
@@ -582,7 +598,8 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   }
 
   @override
-  Future<Timeline> getVMTimeline({int timeOriginMicros, int timeExtentMicros}) {
+  Future<Timeline> getVMTimeline(
+      {int? timeOriginMicros, int? timeExtentMicros}) {
     return _rpcNotSupportedFuture('getVMTimeline');
   }
 
@@ -600,17 +617,12 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   @override
   Future<Response> invoke(
       String isolateId, String targetId, String selector, List argumentIds,
-      {bool disableBreakpoints}) async {
+      {bool? disableBreakpoints}) async {
     await isInitialized;
     _checkIsolate('invoke', isolateId);
     // TODO(798) - respect disableBreakpoints.
-    final remote = await _inspector?.invoke(targetId, selector, argumentIds);
-    final result = _inspector?.instanceRefFor(remote);
-    if (result == null) {
-      throw ChromeDebugException(
-          {'text': 'null result from invoke of $selector'});
-    }
-    return result;
+    final remote = await inspector.invoke(targetId, selector, argumentIds);
+    return _instanceRef(remote);
   }
 
   @override
@@ -668,7 +680,7 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   // signature consistent with the VM service interface.
   @override
   Future<UriList> lookupResolvedPackageUris(String isolateId, List<String> uris,
-      {bool local}) async {
+      {bool? local}) async {
     await isInitialized;
     _checkIsolate('lookupResolvedPackageUris', isolateId);
     return UriList(uris: uris.map(DartUri.toResolvedUri).toList());
@@ -688,7 +700,7 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
 
   @override
   Future<ReloadReport> reloadSources(String isolateId,
-      {bool force, bool pause, String rootLibUri, String packagesUri}) {
+      {bool? force, bool? pause, String? rootLibUri, String? packagesUri}) {
     return Future.error(RPCError(
       'reloadSources',
       RPCError.kMethodNotFound,
@@ -708,9 +720,8 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
 
   @override
   Future<Success> resume(String isolateId,
-      {String step, int frameIndex}) async {
-    if (_inspector == null) throw StateError('No running isolate.');
-    if (_inspector.appConnection.isStarted) {
+      {String? step, int? frameIndex}) async {
+    if (inspector.appConnection.isStarted) {
       return captureElapsedTime(() async {
         await isInitialized;
         _checkIsolate('resume', isolateId);
@@ -718,17 +729,18 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
             .resume(step: step, frameIndex: frameIndex);
       }, (result) => DwdsEvent.resume(step));
     } else {
-      _inspector.appConnection.runMain();
+      inspector.appConnection.runMain();
       return Success();
     }
   }
 
   @override
   Future<Success> setIsolatePauseMode(String isolateId,
-      {String exceptionPauseMode, bool shouldPauseOnExit}) async {
+      {String? exceptionPauseMode, bool? shouldPauseOnExit}) async {
     // TODO(elliette): Is there a way to respect the shouldPauseOnExit parameter
     // in Chrome?
-    return setExceptionPauseMode(isolateId, exceptionPauseMode);
+    return setExceptionPauseMode(
+        isolateId, exceptionPauseMode ?? ExceptionPauseMode.kNone);
   }
 
   @override
@@ -753,8 +765,7 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   Future<Success> setName(String isolateId, String name) async {
     await isInitialized;
     _checkIsolate('setName', isolateId);
-    final isolate = _getIsolate(isolateId);
-    isolate.name = name;
+    inspector.isolate.name = name;
     return Success();
   }
 
@@ -808,9 +819,9 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   StreamController<Event> _chromeConsoleStreamController(
       bool Function(ConsoleAPIEvent) filter,
       {bool includeExceptions = false}) {
-    StreamController<Event> controller;
-    StreamSubscription chromeConsoleSubscription;
-    StreamSubscription exceptionsSubscription;
+    late StreamController<Event> controller;
+    StreamSubscription? chromeConsoleSubscription;
+    StreamSubscription? exceptionsSubscription;
 
     // This is an edge case for this lint apparently
     //
@@ -820,32 +831,32 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
       exceptionsSubscription?.cancel();
     }, onListen: () {
       chromeConsoleSubscription = remoteDebugger.onConsoleAPICalled.listen((e) {
-        final isolate = _inspector?.isolate;
-        if (isolate == null) return;
+        if (!_isIsolateRunning) return;
+        final isolateRef = inspector.isolateRef;
         if (!filter(e)) return;
-        final args = e.params['args'] as List;
-        final item = args[0] as Map;
-        final value = '${item["value"]}\n';
+        final args = e.params?['args'] as List?;
+        final item = args?[0] as Map?;
+        final value = '${item?["value"]}\n';
         controller.add(Event(
             kind: EventKind.kWriteEvent,
             timestamp: DateTime.now().millisecondsSinceEpoch,
-            isolate: _inspector.isolateRef)
+            isolate: isolateRef)
           ..bytes = base64.encode(utf8.encode(value))
           ..timestamp = e.timestamp.toInt());
       });
       if (includeExceptions) {
         exceptionsSubscription =
             remoteDebugger.onExceptionThrown.listen((e) async {
-          final isolate = _inspector?.isolate;
-          if (isolate == null) return;
-          var description = e.exceptionDetails.exception.description;
+          if (!_isIsolateRunning) return;
+          final isolateRef = inspector.isolateRef;
+          var description = e.exceptionDetails.exception?.description;
           if (description != null) {
-            description = await _inspector.mapExceptionStackTrace(description);
+            description = await inspector.mapExceptionStackTrace(description);
           }
           controller.add(Event(
               kind: EventKind.kWriteEvent,
               timestamp: DateTime.now().millisecondsSinceEpoch,
-              isolate: _inspector.isolateRef)
+              isolate: isolateRef)
             ..bytes = base64.encode(utf8.encode(description ?? '')));
         });
       }
@@ -865,9 +876,8 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   /// protocol [Event].
   Future<void> parseDebugEvent(DebugEvent debugEvent) async {
     if (terminatingIsolates) return;
-
-    final isolateRef = _inspector?.isolateRef;
-    if (isolateRef == null) return;
+    if (!_isIsolateRunning) return;
+    final isolateRef = inspector.isolateRef;
 
     _streamNotify(
         EventStreams.kExtension,
@@ -884,14 +894,13 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   /// protocol [Event].
   Future<void> parseRegisterEvent(RegisterEvent registerEvent) async {
     if (terminatingIsolates) return;
+    if (!_isIsolateRunning) return;
 
-    final isolate = _inspector?.isolate;
-    if (isolate == null) return;
+    final isolate = inspector.isolate;
+    final isolateRef = inspector.isolateRef;
     final service = registerEvent.eventData;
-    isolate.extensionRPCs.add(service);
+    isolate.extensionRPCs?.add(service);
 
-    final isolateRef = _inspector?.isolateRef;
-    if (isolateRef == null) return;
     _streamNotify(
         EventStreams.kIsolate,
         Event(
@@ -907,9 +916,9 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
         remoteDebugger.onConsoleAPICalled.listen((event) async {
       if (terminatingIsolates) return;
       if (event.type != 'debug') return;
+      if (!_isIsolateRunning) return;
 
-      final isolate = _inspector?.isolate;
-      if (isolate == null) return;
+      final isolate = inspector.isolate;
       if (isolateRef.id != isolate.id) return;
 
       final firstArgValue = event.args[0].value as String;
@@ -920,7 +929,7 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
           // All inspected objects should be real objects.
           if (event.args[1].type != 'object') break;
 
-          final inspectee = await _inspector.instanceRefFor(event.args[1]);
+          final inspectee = await _instanceRef(event.args[1]);
           _streamNotify(
               EventStreams.kDebug,
               Event(
@@ -946,9 +955,9 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   }
 
   void _handleDeveloperLog(IsolateRef isolateRef, ConsoleAPIEvent event) async {
-    final logObject = event.params['args'][1] as Map;
+    final logObject = event.params?['args'][1] as Map?;
     final logParams = <String, RemoteObject>{};
-    for (dynamic obj in logObject['preview']['properties']) {
+    for (dynamic obj in logObject?['preview']?['properties'] ?? {}) {
       if (obj['name'] != null && obj is Map<String, dynamic>) {
         logParams[obj['name'] as String] = RemoteObject(obj);
       }
@@ -958,12 +967,12 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
       message: await _instanceRef(logParams['message']),
       loggerName: await _instanceRef(logParams['name']),
       level: logParams['level'] != null
-          ? int.tryParse(logParams['level'].value.toString())
+          ? int.tryParse(logParams['level']!.value.toString())
           : 0,
       error: await _instanceRef(logParams['error']),
       time: event.timestamp.toInt(),
       sequenceNumber: logParams['sequenceNumber'] != null
-          ? int.tryParse(logParams['sequenceNumber'].value.toString())
+          ? int.tryParse(logParams['sequenceNumber']!.value.toString())
           : 0,
       stackTrace: await _instanceRef(logParams['stackTrace']),
       zone: await _instanceRef(logParams['zone']),
@@ -1024,12 +1033,9 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
     ]);
   }
 
-  Future<InstanceRef> _instanceRef(RemoteObject obj) async {
-    if (obj == null) {
-      return InstanceHelper.kNullInstanceRef;
-    } else {
-      return _inspector.instanceRefFor(obj);
-    }
+  Future<InstanceRef> _instanceRef(RemoteObject? obj) async {
+    final instance = obj == null ? null : await inspector.instanceRefFor(obj);
+    return instance ?? InstanceHelper.kNullInstanceRef;
   }
 
   static RPCError _rpcNotSupported(String method) {
@@ -1050,7 +1056,7 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
 
   @override
   Future<CpuSamples> getAllocationTraces(String isolateId,
-          {int timeOriginMicros, int timeExtentMicros, String classId}) =>
+          {int? timeOriginMicros, int? timeExtentMicros, String? classId}) =>
       throw UnimplementedError();
 
   @override
@@ -1079,19 +1085,19 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
   ///
   /// This is useful to call at the beginning of API methods that are passed an
   /// isolate id.
-  Isolate _checkIsolate(String methodName, String isolateId) {
-    if (inspector == null) {
-      throw StateError('No inspector set in Domain');
+  Isolate _checkIsolate(String methodName, String? isolateId) {
+    final currentIsolateId = inspector.isolate.id;
+    if (currentIsolateId == null) {
+      throw StateError('No running isolate ID');
     }
-    final isolate = _inspector.isolate;
-    if (isolateId != isolate.id) {
+    if (isolateId != currentIsolateId) {
       _throwSentinel(methodName, SentinelKind.kCollected,
           'Unrecognized isolateId: $isolateId');
     }
-    return isolate;
+    return inspector.isolate;
   }
 
-  static void _throwSentinel(String method, String kind, String message) {
+  static Never _throwSentinel(String method, String kind, String message) {
     final data = <String, String>{'kind': kind, 'valueAsString': message};
     throw SentinelException.parse(method, data);
   }
diff --git a/dwds/lib/src/services/expression_evaluator.dart b/dwds/lib/src/services/expression_evaluator.dart
index 9b6ff6809..71e9c5d2e 100644
--- a/dwds/lib/src/services/expression_evaluator.dart
+++ b/dwds/lib/src/services/expression_evaluator.dart
@@ -74,7 +74,7 @@ class ExpressionEvaluator {
   /// [expression] dart expression to evaluate.
   Future<RemoteObject> evaluateExpression(
     String isolateId,
-    String libraryUri,
+    String? libraryUri,
     String expression,
     Map<String, String>? scope,
   ) async {
@@ -84,6 +84,10 @@ class ExpressionEvaluator {
       return _createError(ErrorKind.invalidInput, expression);
     }
 
+    if (libraryUri == null) {
+      return _createError(ErrorKind.invalidInput, 'no library uri');
+    }
+
     final module = await _modules.moduleForlibrary(libraryUri);
     if (module == null) {
       return _createError(ErrorKind.internal, 'no module for $libraryUri');
diff --git a/dwds/lib/src/utilities/domain.dart b/dwds/lib/src/utilities/domain.dart
index 48304e2a4..5cf91a15a 100644
--- a/dwds/lib/src/utilities/domain.dart
+++ b/dwds/lib/src/utilities/domain.dart
@@ -98,7 +98,7 @@ abstract class AppInspectorInterface {
   });
 
   /// Lookup current isolate's memory usage statistics.
-  Future<MemoryUsage?> getMemoryUsage();
+  Future<MemoryUsage> getMemoryUsage();
 }
 
 /// A common superclass to allow implementations of different parts of the

From c835a40ac4c12ce7da397072fcbeff778850c315 Mon Sep 17 00:00:00 2001
From: Anna Gringauze <annagrin@google.com>
Date: Mon, 18 Jul 2022 12:30:50 -0700
Subject: [PATCH 6/7] Made  return a non-null value

---
 dwds/lib/src/debugging/inspector.dart           | 14 +++++++++-----
 dwds/lib/src/services/chrome_proxy_service.dart |  8 +-------
 dwds/lib/src/utilities/domain.dart              |  2 +-
 3 files changed, 11 insertions(+), 13 deletions(-)

diff --git a/dwds/lib/src/debugging/inspector.dart b/dwds/lib/src/debugging/inspector.dart
index 816f938fa..4a9f7a72a 100644
--- a/dwds/lib/src/debugging/inspector.dart
+++ b/dwds/lib/src/debugging/inspector.dart
@@ -362,7 +362,7 @@ class AppInspector implements AppInspectorInterface {
   }
 
   @override
-  Future<Obj?> getObject(String objectId, {int? offset, int? count}) async {
+  Future<Obj> getObject(String objectId, {int? offset, int? count}) async {
     try {
       final library = await getLibrary(objectId);
       if (library != null) {
@@ -389,17 +389,21 @@ class AppInspector implements AppInspectorInterface {
         'are supported for getObject');
   }
 
-  Future<Script?> _getScript(ScriptRef scriptRef) async {
+  Future<Script> _getScript(ScriptRef scriptRef) async {
     final libraryId = _scriptIdToLibraryId[scriptRef.id];
     final scriptUri = scriptRef.uri;
     final scriptId = scriptRef.id;
-    if (libraryId == null || scriptUri == null || scriptId == null) return null;
-
+    if (libraryId == null || scriptUri == null || scriptId == null) {
+      throwInvalidParam(
+          'getObject',
+          'No source for script $scriptRef with '
+              'scriptId: $scriptId, libraryId: $libraryId, scriptUri: $scriptUri');
+    }
     final serverPath = DartUri(scriptUri, _root).serverPath;
     final source = await _assetReader.dartSourceContents(serverPath);
     if (source == null) {
       throwInvalidParam('getObject',
-          'No source for script $scriptId (server path: $serverPath)');
+          'No source for script $scriptRef with server path: $serverPath');
     }
     return Script(
         uri: scriptRef.uri,
diff --git a/dwds/lib/src/services/chrome_proxy_service.dart b/dwds/lib/src/services/chrome_proxy_service.dart
index 8f95a8184..1ef2421a2 100644
--- a/dwds/lib/src/services/chrome_proxy_service.dart
+++ b/dwds/lib/src/services/chrome_proxy_service.dart
@@ -533,13 +533,7 @@ ${globalLoadStrategy.loadModuleSnippet}("dart_sdk").developer.invokeExtension(
       {int? offset, int? count}) async {
     await isInitialized;
     _checkIsolate('getObject', isolateId);
-    final result =
-        await inspector.getObject(objectId, offset: offset, count: count);
-    if (result == null) {
-      throw RPCError('getObject', RPCError.kInternalError,
-          'failed to find object for $objectId: null result');
-    }
-    return result;
+    return inspector.getObject(objectId, offset: offset, count: count);
   }
 
   @override
diff --git a/dwds/lib/src/utilities/domain.dart b/dwds/lib/src/utilities/domain.dart
index 9829af561..a41e72129 100644
--- a/dwds/lib/src/utilities/domain.dart
+++ b/dwds/lib/src/utilities/domain.dart
@@ -76,7 +76,7 @@ abstract class AppInspectorInterface {
       {bool returnByValue = false, bool awaitPromise = false});
 
   /// Lookup an `object` from some isolate by its [objectId].
-  Future<Obj?> getObject(String objectId, {int offset, int count});
+  Future<Obj> getObject(String objectId, {int offset, int count});
 
   /// All the scripts in the isolate.
   Future<ScriptList> getScripts();

From 965e8f8b73c868a9a1075f8cc79b58c8a6e8806f Mon Sep 17 00:00:00 2001
From: Anna Gringauze <annagrin@google.com>
Date: Mon, 18 Jul 2022 13:22:58 -0700
Subject: [PATCH 7/7] Cleanup error messages in Inspector._getScript

---
 dwds/lib/src/debugging/inspector.dart | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/dwds/lib/src/debugging/inspector.dart b/dwds/lib/src/debugging/inspector.dart
index 4a9f7a72a..d727035b4 100644
--- a/dwds/lib/src/debugging/inspector.dart
+++ b/dwds/lib/src/debugging/inspector.dart
@@ -390,20 +390,21 @@ class AppInspector implements AppInspectorInterface {
   }
 
   Future<Script> _getScript(ScriptRef scriptRef) async {
-    final libraryId = _scriptIdToLibraryId[scriptRef.id];
-    final scriptUri = scriptRef.uri;
     final scriptId = scriptRef.id;
-    if (libraryId == null || scriptUri == null || scriptId == null) {
-      throwInvalidParam(
-          'getObject',
-          'No source for script $scriptRef with '
-              'scriptId: $scriptId, libraryId: $libraryId, scriptUri: $scriptUri');
+    final scriptUri = scriptRef.uri;
+    if (scriptId == null || scriptUri == null) {
+      throwInvalidParam('getObject', 'No script info for script $scriptRef');
     }
     final serverPath = DartUri(scriptUri, _root).serverPath;
     final source = await _assetReader.dartSourceContents(serverPath);
     if (source == null) {
       throwInvalidParam('getObject',
-          'No source for script $scriptRef with server path: $serverPath');
+          'No source for $scriptRef  with serverPath: $serverPath');
+    }
+    final libraryId = _scriptIdToLibraryId[scriptId];
+    if (libraryId == null) {
+      throwInvalidParam('getObject',
+          'No library for script $scriptRef with libraryId: $libraryId');
     }
     return Script(
         uri: scriptRef.uri,