migrate package:frontend_server/compute_kernel.dart to null safety

unblocks dart-lang/language#2274

Bug: #49212
Change-Id: I8d6b055dbc00dadce5efee57692ee1f21e8fde77
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/247601
Commit-Queue: Jake Macdonald <jakemac@google.com>
Auto-Submit: Jake Macdonald <jakemac@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

pkg/frontend_server/lib/compute_kernel.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 = 2.8
-
 /// A library to invoke the CFE to compute kernel summary files.
 ///
 /// Used by `utils/bazel/kernel_worker.dart`.
@@ -119,7 +117,7 @@ final summaryArgsParser = new ArgParser()
 
 class ComputeKernelResult {
   final bool succeeded;
-  final fe.InitializedCompilerState previousState;
+  final fe.InitializedCompilerState? previousState;
 
   ComputeKernelResult(this.succeeded, this.previousState);
 }
@@ -134,9 +132,9 @@ class ComputeKernelResult {
 /// Returns whether or not the summary was successfully output.
 Future<ComputeKernelResult> computeKernel(List<String> args,
     {bool isWorker: false,
-    StringBuffer outputBuffer,
-    Map<Uri, List<int>> inputDigests,
-    fe.InitializedCompilerState previousState}) async {
+    StringBuffer? outputBuffer,
+    Map<Uri, List<int>>? inputDigests,
+    fe.InitializedCompilerState? previousState}) async {
   inputDigests ??= <Uri, List<int>>{};
   dynamic out = outputBuffer ?? stderr;
   bool succeeded = true;
@@ -172,7 +170,7 @@ Future<ComputeKernelResult> computeKernel(List<String> args,
   // TODO(sigmund,jakemac): make target mandatory. We allow null to be backwards
   // compatible while we migrate existing clients of this tool.
   var targetName =
-      (parsedArgs['target'] as String) ?? (summaryOnly ? 'ddc' : 'vm');
+      (parsedArgs['target'] as String?) ?? (summaryOnly ? 'ddc' : 'vm');
   var targetFlags = new TargetFlags(
       trackWidgetCreation: trackWidgetCreation,
       enableNullSafety: nnbdMode == fe.NnbdMode.Strong);
@@ -225,6 +223,7 @@ Future<ComputeKernelResult> computeKernel(List<String> args,
       break;
     default:
       out.writeln('error: unsupported target: $targetName');
+      return ComputeKernelResult(false, previousState);
   }
 
   List<Uri> linkedInputs =
@@ -360,22 +359,22 @@ Future<ComputeKernelResult> computeKernel(List<String> args,
     }
   }
 
-  List<int> kernel;
+  List<int>? kernel;
   bool wroteUsedDills = false;
   if (usingIncrementalCompiler) {
     state.options.onDiagnostic = onDiagnostic;
     IncrementalCompilerResult incrementalCompilerResult =
-        await state.incrementalCompiler.computeDelta(
+        await state.incrementalCompiler!.computeDelta(
             entryPoints: sources,
             fullComponent: true,
             trackNeededDillLibraries: recordUsedInputs);
     Component incrementalComponent = incrementalCompilerResult.component;
 
     if (recordUsedInputs) {
       Set<Uri> usedOutlines = {};
-      for (Library lib in incrementalCompilerResult.neededDillLibraries) {
+      for (Library lib in incrementalCompilerResult.neededDillLibraries!) {
         if (lib.importUri.isScheme("dart")) continue;
-        Uri uri = state.libraryToInputDill[lib.importUri];
+        Uri? uri = state.libraryToInputDill![lib.importUri];
         if (uri == null) {
           throw new StateError("Library ${lib.importUri} was recorded as used, "
               "but was not in the list of known libraries.");
@@ -388,7 +387,7 @@ Future<ComputeKernelResult> computeKernel(List<String> args,
       wroteUsedDills = true;
     }
 
-    kernel = await state.incrementalCompiler.context.runInContext((_) {
+    kernel = await state.incrementalCompiler!.context.runInContext((_) {
       if (summaryOnly) {
         incrementalComponent.uriToSource.clear();
         incrementalComponent.problemsAsJson = null;
@@ -412,13 +411,15 @@ Future<ComputeKernelResult> computeKernel(List<String> args,
     kernel = await fe.compileSummary(state, sources, onDiagnostic,
         includeOffsets: false);
   } else {
-    Component component = await fe
+    Component? component = await fe
         .compileComponent(state, sources, onDiagnostic, buildSummary: summary);
-    kernel = fe.serializeComponent(component,
-        filter: excludeNonSources
-            ? (library) => sources.contains(library.importUri)
-            : null,
-        includeOffsets: true);
+    if (component != null) {
+      kernel = fe.serializeComponent(component,
+          filter: excludeNonSources
+              ? (library) => sources.contains(library.importUri)
+              : null,
+          includeOffsets: true);
+    }
   }
   state.options.onDiagnostic = null; // See http://dartbug.com/36983.
 
@@ -468,7 +469,6 @@ class DevCompilerSummaryTarget extends DevCompilerTarget with SummaryMixin {
 }
 
 Uri toUri(String uriString) {
-  if (uriString == null) return null;
   // Windows-style paths use '\', so convert them to '/' in case they've been
   // concatenated with Unix-style paths.
   return Uri.base.resolve(uriString.replaceAll("\\", "/"));

pkg/frontend_server/lib/src/strong_components.dart

@@ -2,7 +2,6 @@
 // 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:kernel/ast.dart';
 import 'package:kernel/util/graph.dart';
 
@@ -45,7 +44,7 @@ class StrongComponents {
   final Uri mainUri;
 
   /// The filesystem instance for resolving files.
-  final FileSystem fileSystem;
+  final FileSystem? fileSystem;
 
   /// The set of libraries for each module URI.
   ///
@@ -68,10 +67,13 @@ class StrongComponents {
     }
     // If we don't have a file uri, just use the first library in the
     // component.
-    Library entrypoint = component.libraries.firstWhere(
-        (Library library) =>
-            library.fileUri == mainUri || library.importUri == mainUri,
-        orElse: () => null);
+    Library? entrypoint;
+    for (Library library in component.libraries) {
+      if (library.fileUri == mainUri || library.importUri == mainUri) {
+        entrypoint = library;
+        break;
+      }
+    }
 
     if (entrypoint == null) {
       throw Exception('Could not find entrypoint ${mainUri} in Component.');

pkg/frontend_server/test/src/strong_components_test.dart

@@ -2,7 +2,6 @@
 // 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:frontend_server/src/strong_components.dart';
 import 'package:kernel/ast.dart';
 import 'package:test/test.dart';