auto-submit · Oct 1, 2026 · Sep 24, 2026 · Sep 24, 2026 · Sep 24, 2026 · Sep 25, 2026
diff --git a/.github/workflows/roll-dart-dependencies.yml b/.github/workflows/roll-dart-dependencies.yml
@@ -95,8 +95,8 @@ jobs:
         env:
           DART_HASH: ${{ inputs.dart_hash }}
         run: |
-          # Check if the root DEPS file has changed
-          if git diff --exit-code DEPS; then
+          # Check if the root DEPS file or supports_dart2wasm.js has changed
+          if git diff --exit-code DEPS engine/src/flutter/lib/web_ui/flutter_js/src/supports_dart2wasm.js; then
             echo "changed=false" >> "$GITHUB_OUTPUT"
             echo "## ✅ Dart version already up to date" >> "$GITHUB_STEP_SUMMARY"
             echo "The DEPS file is already up to date with Dart hash \`$DART_HASH\`." >> "$GITHUB_STEP_SUMMARY"
@@ -108,7 +108,7 @@ jobs:
         if: steps.git-check.outputs.changed == 'true'
         run: |
           gh auth setup-git
-          git add DEPS
+          git add DEPS engine/src/flutter/lib/web_ui/flutter_js/src/supports_dart2wasm.js
           git commit -m "Update flutter DEPS to dart $DART_HASH"
           git push origin "$BRANCH_NAME" --force
 

diff --git a/engine/src/flutter/ci/test/create_updated_flutter_deps_test.dart b/engine/src/flutter/ci/test/create_updated_flutter_deps_test.dart
@@ -0,0 +1,26 @@
+// Copyright 2013 The Flutter Authors. 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:io' as io;
+
+import 'package:engine_repo_tools/engine_repo_tools.dart';
+import 'package:path/path.dart' as path;
+import 'package:test/test.dart';
+
+void main() {
+  test('create_updated_flutter_deps_tests.py passes and verifies dart2wasm support parity', () {
+    final io.Directory srcDir = Engine.findWithin().srcDir;
+    final String testScript = path.join(
+      srcDir.path,
+      'tools',
+      'dart',
+      'create_updated_flutter_deps_tests.py',
+    );
+    final python = io.Platform.isWindows ? 'python' : 'python3';
+    final io.ProcessResult result = io.Process.runSync(python, <String>[
+      testScript,
+    ], workingDirectory: srcDir.parent.path);
+    expect(result.exitCode, 0, reason: 'stdout:\n${result.stdout}\nstderr:\n${result.stderr}');
+  });
+}
diff --git a/engine/src/flutter/lib/web_ui/flutter_js/sources.gni b/engine/src/flutter/lib/web_ui/flutter_js/sources.gni
@@ -11,6 +11,7 @@ flutter_js_source_list = [
   "src/loader.js",
   "src/service_worker_loader.js",
   "src/skwasm_loader.js",
+  "src/supports_dart2wasm.js",
   "src/trusted_types.js",
   "src/utils.js",
 

diff --git a/engine/src/flutter/lib/web_ui/flutter_js/src/browser_environment.js b/engine/src/flutter/lib/web_ui/flutter_js/src/browser_environment.js
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import { supportsDart2Wasm as defaultSupportsDart2Wasm } from './supports_dart2wasm.js';
+
 /** @type {import("./types").WasmAllowList} */
 export const defaultWasmSupport = {
   "blink": true,
@@ -64,16 +66,7 @@ const supportsDart2Wasm = () => {
   //      https://bugzilla.mozilla.org/show_bug.cgi?id=2006811
   if (browserEngine === "gecko" && getFirefoxVersion() < 147) return false;
 
-  // The `<app>.support.js` expression emitted by 
-  // ```
-  //   % dart compile wasm \
-  //          --extra-compiler-option=--require-js-string-builtin \
-  //          -o hello.wasm \
-  //          hello.dart
-  //   % cat hello.support.js
-  // ```
-  // It checks support for Wasm GC, SIMD, `js-string` builtins, and `try_table`.
-  return (WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,5,1,95,1,120,0]))&&WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,5,1,96,0,1,123,3,2,1,0,10,10,1,8,0,65,0,253,15,253,98,11]))&&!WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,4,1,96,0,0,2,23,1,14,119,97,115,109,58,106,115,45,115,116,114,105,110,103,4,99,97,115,116,0,0]),{"builtins":["js-string"]})&&WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,4,1,96,0,0,3,2,1,0,10,13,1,11,0,2,64,31,64,1,2,0,11,11,11])));
+  return window._flutter?.supportsDart2Wasm ?? defaultSupportsDart2Wasm();
 }
 
 const detectWebGLVersion = () => {

diff --git a/engine/src/flutter/lib/web_ui/flutter_js/src/loader.js b/engine/src/flutter/lib/web_ui/flutter_js/src/loader.js
@@ -87,7 +87,7 @@ export class FlutterLoader {
       switch (renderer) {
         case "skwasm":
           if (!supportsDart2Wasm) {
-            return "Skwasm requires WasmGC support; this browser does not implement it yet.";
+            return "Skwasm requires WebAssembly features (WasmGC, SIMD, js-string builtins, try_table) that this browser does not support.";
           }
           if (!(browserEnvironment.webGLVersion > 0)) {
             return "Skwasm requires WebGL support; this browser does not provide it.";
@@ -106,7 +106,7 @@ export class FlutterLoader {
     const buildIncompatibilityReason = (build) => {
       if (build.compileTarget === "dart2wasm") {
         if (!supportsDart2Wasm) {
-          return "dart2wasm requires WasmGC support; this browser does not implement it yet.";
+          return "dart2wasm requires WebAssembly features (WasmGC, SIMD, js-string builtins, try_table) that this browser does not support.";
         }
         if (!enableWasm) {
           if (config.wasmAllowList?.[browserEnvironment.browserEngine] === false) {

diff --git a/engine/src/flutter/lib/web_ui/flutter_js/src/supports_dart2wasm.js b/engine/src/flutter/lib/web_ui/flutter_js/src/supports_dart2wasm.js
@@ -0,0 +1,12 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// GENERATED FILE. DO NOT EDIT.
+//
+// Generated by `engine/src/tools/dart/create_updated_flutter_deps.py` from
+// `pkg/dart2wasm/lib/compile.dart`.
+
+export const supportsDart2Wasm = () => {
+  return (WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,5,1,95,1,120,0]))&&WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,5,1,96,0,1,123,3,2,1,0,10,10,1,8,0,65,0,253,15,253,98,11]))&&!WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,4,1,96,0,0,2,23,1,14,119,97,115,109,58,106,115,45,115,116,114,105,110,103,4,99,97,115,116,0,0]),{"builtins":["js-string"]})&&WebAssembly.validate(new Uint8Array([0,97,115,109,1,0,0,0,1,4,1,96,0,0,3,2,1,0,10,13,1,11,0,2,64,31,64,1,2,0,11,11,11])));
+};
diff --git a/engine/src/flutter/lib/web_ui/test/fallbacks/fallbacks_test.dart b/engine/src/flutter/lib/web_ui/test/fallbacks/fallbacks_test.dart
@@ -28,6 +28,12 @@ external JSAny? get _flutterBuildConfig;
 @JS('_flutter.buildConfig')
 external set _flutterBuildConfig(JSAny? value);
 
+@JS('_flutter.supportsDart2Wasm')
+external JSAny? get _flutterSupportsDart2Wasm;
+
+@JS('_flutter.supportsDart2Wasm')
+external set _flutterSupportsDart2Wasm(JSAny? value);
+
 @JS('WebAssembly.validate')
 external JSFunction get _wasmValidate;
 
@@ -52,6 +58,7 @@ Future<void> testMain() async {
 
   test('loader strictly honors wasmAllowList and WasmGC capability for dart2wasm builds', () async {
     final JSAny? originalBuildConfig = _flutterBuildConfig;
+    final JSAny? originalSupportsDart2Wasm = _flutterSupportsDart2Wasm;
     final JSFunction originalValidate = _wasmValidate;
     try {
       _flutterBuildConfig = <String, Object?>{
@@ -87,9 +94,10 @@ Future<void> testMain() async {
         ),
       );
 
-      // 2. When WasmGC capability validation fails, explicit opt-in via
-      // wasmAllowList must still reject the dart2wasm build.
-      _wasmValidate = (() => false).toJS;
+      // 2. When _flutter.supportsDart2Wasm is false (e.g. from main.dart.support.js),
+      // explicit opt-in via wasmAllowList must reject the dart2wasm build even if
+      // defaultSupportsDart2Wasm() would pass.
+      _flutterSupportsDart2Wasm = false.toJS;
       final JSAny? optInOptions = <String, Object?>{
         'config': <String, Object?>{
           'wasmAllowList': <String, bool>{
@@ -109,7 +117,22 @@ Future<void> testMain() async {
           ),
         ),
       );
+
+      // 3. When _flutter.supportsDart2Wasm is unset and WebAssembly.validate fails,
+      // explicit opt-in via wasmAllowList must still reject the dart2wasm build.
+      _flutterSupportsDart2Wasm = null;
+      _wasmValidate = (() => false).toJS;
+
+      await expectLater(
+        _flutterLoaderLoad(optInOptions).toDart,
+        throwsA(
+          predicate<Object>(
+            (Object e) => e.toString().contains('FlutterLoader could not find a build compatible'),
+          ),
+        ),
+      );
     } finally {
+      _flutterSupportsDart2Wasm = originalSupportsDart2Wasm;
       _wasmValidate = originalValidate;
       _flutterBuildConfig = originalBuildConfig;
     }