Check that ErrorWidget.builder is not modified after test (#30898)

packages/flutter/test/widgets/error_widget_builder_test.dart

@@ -7,6 +7,7 @@ import 'package:flutter/widgets.dart';
 
 void main() {
   testWidgets('ErrorWidget.builder', (WidgetTester tester) async {
+    final ErrorWidgetBuilder oldBuilder = ErrorWidget.builder;
     ErrorWidget.builder = (FlutterErrorDetails details) {
       return const Text('oopsie!', textDirection: TextDirection.ltr);
     };
@@ -21,5 +22,6 @@ void main() {
     );
     expect(tester.takeException().toString(), 'test');
     expect(find.text('oopsie!'), findsOneWidget);
+    ErrorWidget.builder = oldBuilder;
   });
 }

packages/flutter_test/lib/src/binding.dart

@@ -584,6 +584,7 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
 
     final bool autoUpdateGoldensBeforeTest = autoUpdateGoldenFiles;
     final TestExceptionReporter reportTestExceptionBeforeTest = reportTestException;
+    final ErrorWidgetBuilder errorWidgetBuilderBeforeTest = ErrorWidget.builder;
 
     // run the test
     await testBody();
@@ -598,6 +599,7 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
       invariantTester();
       _verifyAutoUpdateGoldensUnset(autoUpdateGoldensBeforeTest);
       _verifyReportTestExceptionUnset(reportTestExceptionBeforeTest);
+      _verifyErrorWidgetBuilderUnset(errorWidgetBuilderBeforeTest);
       _verifyInvariants();
     }
 
@@ -667,6 +669,21 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
     }());
   }
 
+  void _verifyErrorWidgetBuilderUnset(ErrorWidgetBuilder valueBeforeTest) {
+    assert(() {
+      if (ErrorWidget.builder != valueBeforeTest) {
+        FlutterError.reportError(FlutterErrorDetails(
+          exception: FlutterError(
+              'The value of ErrorWidget.builder was changed by the test.',
+          ),
+          stack: StackTrace.current,
+          library: 'Flutter test framework',
+        ));
+      }
+      return true;
+    }());
+  }
+
   /// Called by the [testWidgets] function after a test is executed.
   void postTest() {
     assert(inTest);