[web] Fix assertion thrown when hot restarting during animation (#175856)

Fixes https://github.com/flutter/flutter/issues/140684
Fixes https://github.com/flutter/flutter/issues/175260

engine/src/flutter/lib/web_ui/lib/src/engine/frame_service.dart

@@ -10,6 +10,7 @@ import 'package:ui/ui.dart' as ui;
 
 import 'dom.dart';
 import 'frame_timing_recorder.dart';
+import 'initialization.dart';
 import 'platform_dispatcher.dart';
 
 /// Provides frame scheduling functionality and frame lifecycle information to
@@ -18,6 +19,10 @@ import 'platform_dispatcher.dart';
 /// If new frame-related functionality needs to be added to the web engine,
 /// prefer to add it here instead of implementing it ad hoc.
 class FrameService {
+  FrameService() {
+    registerHotRestartListener(_dispose);
+  }
+
   /// The singleton instance of the [FrameService] used to schedule frames.
   ///
   /// This may be overridden in tests, for example, to pump fake frames, using
@@ -25,6 +30,8 @@ class FrameService {
   static FrameService get instance => _instance ??= FrameService();
   static FrameService? _instance;
 
+  bool _isDisposed = false;
+
   /// Overrides the value returned by [instance].
   ///
   /// If [mock] is null, resets the value of [instance] back to real
@@ -98,6 +105,18 @@ class FrameService {
       //   functionality that may throw exceptions, or produce wasm traps.
       _isFrameScheduled = false;
 
+      if (_isDisposed) {
+        // Skip this animation frame because the instance has been disposed, meaning there was a
+        // hot restart performed. During a hot restart, Dart automatically cancels timers and
+        // microtasks, but animation frames are requested directly from the browser which isn't
+        // aware of hot restarts, and that leads to problems.
+        //
+        // See:
+        // - https://github.com/flutter/flutter/issues/175260
+        // - https://github.com/flutter/flutter/issues/140684#issuecomment-3251179364
+        return;
+      }
+
       try {
         _isRenderingFrame = true;
         _frameData = ui.FrameData(frameNumber: _frameData.frameNumber + 1);
@@ -196,4 +215,11 @@ class FrameService {
       EnginePlatformDispatcher.instance.invokeOnDrawFrame();
     }
   }
+
+  void _dispose() {
+    if (identical(this, _instance)) {
+      _instance = null;
+    }
+    _isDisposed = true;
+  }
 }

engine/src/flutter/lib/web_ui/test/engine/frame_service_test.dart

@@ -14,12 +14,18 @@ void main() {
 
 void testMain() {
   group('FrameService', () {
-    setUp(() {
+    void resetFrameService() {
+      // Emulate a hot restart to clear listeners from previous tests.
+      debugEmulateHotRestart();
+
       FrameService.debugOverrideFrameService(null);
       expect(FrameService.instance.runtimeType, FrameService);
       EnginePlatformDispatcher.instance.onBeginFrame = null;
       EnginePlatformDispatcher.instance.onDrawFrame = null;
-    });
+    }
+
+    setUp(resetFrameService);
+    tearDownAll(resetFrameService);
 
     test('instance is valid and can be overridden', () {
       final defaultInstance = FrameService.instance;
@@ -151,6 +157,40 @@ void testMain() {
       expect(isRenderingInOnDrawFrame, isTrue);
       expect(valueInOnFinishedRenderingFrame, isFalse);
     });
+
+    test('Frame is cancelled after a hot restart', () async {
+      final instance = FrameService.instance;
+
+      final frameCompleter = Completer<void>();
+      instance.onFinishedRenderingFrame = () {
+        frameCompleter.complete();
+      };
+
+      expect(instance.isFrameScheduled, isFalse);
+      instance.scheduleFrame();
+      expect(instance.isFrameScheduled, isTrue);
+      // Perform a hot restart immediately after scheduling the frame.
+      debugEmulateHotRestart();
+
+      // Wait for 1 second for the frame to be rendered.
+      bool timedOut = false;
+      await frameCompleter.future.timeout(
+        const Duration(seconds: 1),
+        onTimeout: () {
+          timedOut = true;
+        },
+      );
+      // No frame should've been rendered because the animation frame callback should've been
+      // cancelled on hot restart.
+      expect(timedOut, isTrue);
+      expect(instance.frameData.frameNumber, isZero);
+      expect(frameCompleter.isCompleted, isFalse);
+      // ... and no frame is scheduled.
+      expect(instance.isFrameScheduled, isFalse);
+
+      // To avoid leaving an uncompleted completer, let's complete it.
+      frameCompleter.complete();
+    });
   });
 }