Fix bug thattimeDilation is not reset, causing subsequent test errors, and add verifications to ensure such problem does not exist in the future (#113830)

dev/tools/examples_smoke_test.dart

@@ -119,6 +119,7 @@ Future<File> generateTest(Directory apiDir) async {
   // Collect the examples, and import them all as separate symbols.
   final List<String> imports = <String>[];
   imports.add('''import 'package:flutter/widgets.dart';''');
+  imports.add('''import 'package:flutter/scheduler.dart';''');
   imports.add('''import 'package:flutter_test/flutter_test.dart';''');
   imports.add('''import 'package:integration_test/integration_test.dart';''');
   final List<ExampleInfo> infoList = <ExampleInfo>[];
@@ -165,6 +166,7 @@ void main() {
         expect(find.byType(WidgetsApp), findsOneWidget);
       } finally {
         ErrorWidget.builder = originalBuilder;
+        timeDilation = 1.0;
       }
     },
   );

packages/flutter/lib/src/scheduler/binding.dart

@@ -652,6 +652,20 @@ mixin SchedulerBinding on BindingBase {
     return true;
   }
 
+  /// Asserts that there is no artificial time dilation in debug mode.
+  ///
+  /// Throws a [FlutterError] if there are such dilation, as this will make
+  /// subsequent tests see dilation and thus flaky.
+  bool debugAssertNoTimeDilation(String reason) {
+    assert(() {
+      if (timeDilation != 1.0) {
+        throw FlutterError(reason);
+      }
+      return true;
+    }());
+    return true;
+  }
+
   /// Prints the stack for where the current transient callback was registered.
   ///
   /// A transient frame callback is one that was registered with

packages/flutter/test/scheduler/binding_test.dart

@@ -13,4 +13,19 @@ void main() {
     SchedulerBinding.instance.scheduleForcedFrame();
     expect(SchedulerBinding.instance.platformDispatcher.onBeginFrame, isNotNull);
   });
+
+  test('debugAssertNoTimeDilation does not throw if time dilate already reset', () async {
+    timeDilation = 2.0;
+    timeDilation = 1.0;
+    SchedulerBinding.instance.debugAssertNoTimeDilation('reason'); // no error
+  });
+
+  test('debugAssertNoTimeDilation throw if time dilate not reset', () async {
+    timeDilation = 3.0;
+    expect(
+      () => SchedulerBinding.instance.debugAssertNoTimeDilation('reason'),
+      throwsA(isA<FlutterError>().having((FlutterError e) => e.message, 'message', 'reason')),
+    );
+    timeDilation = 1.0;
+  });
 }

packages/flutter/test/scheduler/scheduler_test.dart

@@ -215,6 +215,8 @@ void main() {
     tick(const Duration(seconds: 8));
     expect(lastTimeStamp, const Duration(seconds: 3)); // 2s + (8 - 6)s / 2
     expect(lastSystemTimeStamp, const Duration(seconds: 8));
+
+    timeDilation = 1.0; // restore time dilation, or it will affect other tests
   });
 
   test('Animation frame scheduled in the middle of the warm-up frame', () {

packages/flutter/test/scheduler/ticker_test.dart

@@ -124,6 +124,8 @@ void main() {
     expect(lastDuration, const Duration(milliseconds: 20));
 
     ticker.dispose();
+
+    timeDilation = 1.0; // restore time dilation, or it will affect other tests
   });
 
   testWidgets('Ticker can be slowed down with time dilation', (WidgetTester tester) async {
@@ -140,6 +142,8 @@ void main() {
     expect(lastDuration, const Duration(milliseconds: 5));
 
     ticker.dispose();
+
+    timeDilation = 1.0; // restore time dilation, or it will affect other tests
   });
 
   testWidgets('Ticker stops ticking when application is paused', (WidgetTester tester) async {

packages/flutter_test/lib/src/binding.dart

@@ -965,6 +965,9 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
     assert(debugAssertNoPendingPerformanceModeRequests(
       'A performance mode was requested and not disposed by a test.'
     ));
+    assert(debugAssertNoTimeDilation(
+      'The timeDilation was changed and not reset by the test.'
+    ));
     assert(debugAssertAllFoundationVarsUnset(
       'The value of a foundation debug variable was changed by the test.',
       debugPrintOverride: debugPrintOverride,