Throw an exception when a wasm trap is hit

wasm/lib/src/runtime.g.dart

@@ -597,9 +597,8 @@ class WasmRuntime {
       calloc.free(trapMessage);
       final entry = _traps.remove(message);
       if (entry == null) {
-        throw WasmError(
-          'This case is not (yet) supported. Please file an issue on pkg:wasm.',
-        );
+        // TODO(#87): Report a full stack trace to the user.
+        throw WasmException(message);
       }
       // ignore: only_throw_errors
       throw entry.exception;

wasm/lib/src/wasm_error.dart

@@ -12,3 +12,14 @@ class WasmError extends Error {
   @override
   String toString() => 'WasmError: $message';
 }
+
+/// Exception that wraps exceptions (traps) thrown inside wasm code.
+class WasmException implements Exception {
+  /// Describes the nature of the exception.
+  final String message;
+
+  WasmException(this.message);
+
+  @override
+  String toString() => 'WasmException: $message';
+}

wasm/test/test_shared.dart

@@ -8,3 +8,8 @@ import 'package:wasm/src/wasm_error.dart';
 Matcher throwsWasmError(Object messageMatcher) => throwsA(
       isA<WasmError>().having((p0) => p0.message, 'message', messageMatcher),
     );
+
+Matcher throwsWasmException(Object messageMatcher) => throwsA(
+      isA<WasmException>()
+          .having((p0) => p0.message, 'message', messageMatcher),
+    );

wasm/test/trap_test.dart

@@ -0,0 +1,33 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+// Test that we can load a wasm module, find a function, and call it.
+import 'dart:typed_data';
+
+import 'package:test/test.dart';
+import 'package:wasm/wasm.dart';
+
+import 'test_shared.dart';
+
+void main() {
+  test('basics', () {
+    // (func $foo (export "foo") (type $t0) (unreachable))
+    final data = Uint8List.fromList([
+      0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00, 0x01, 0x04, 0x01, 0x60, //
+      0x00, 0x00, 0x03, 0x02, 0x01, 0x00, 0x04, 0x05, 0x01, 0x70, 0x01, 0x07,
+      0x07, 0x05, 0x03, 0x01, 0x00, 0x11, 0x07, 0x10, 0x02, 0x03, 0x66, 0x6f,
+      0x6f, 0x00, 0x00, 0x06, 0x6d, 0x65, 0x6d, 0x6f, 0x72, 0x79, 0x02, 0x00,
+      0x0a, 0x05, 0x01, 0x03, 0x00, 0x00, 0x0b, 0x00, 0x2b, 0x04, 0x6e, 0x61,
+      0x6d, 0x65, 0x01, 0x06, 0x01, 0x00, 0x03, 0x66, 0x6f, 0x6f, 0x02, 0x03,
+      0x01, 0x00, 0x00, 0x04, 0x05, 0x01, 0x00, 0x02, 0x74, 0x30, 0x05, 0x05,
+      0x01, 0x00, 0x02, 0x54, 0x30, 0x06, 0x09, 0x01, 0x00, 0x06, 0x6d, 0x65,
+      0x6d, 0x6f, 0x72, 0x79,
+    ]);
+
+    final inst = WasmModule(data).builder().build();
+    final foo = inst.lookupFunction('foo');
+    // ignore: unnecessary_lambdas
+    expect(() => foo(), throwsWasmException('unreachable'));
+  });
+}

wasm/tool/runtime_template.dart.t

@@ -118,9 +118,8 @@ class WasmRuntime {
       calloc.free(trapMessage);
       final entry = _traps.remove(message);
       if (entry == null) {
-        throw WasmError(
-          'This case is not (yet) supported. Please file an issue on pkg:wasm.',
-        );
+        // TODO(#87): Report a full stack trace to the user.
+        throw WasmException(message);
       }
       // ignore: only_throw_errors
       throw entry.exception;