Add tests for scaffold messenger examples (flutter#152536)

Contributes to flutter#130459

It adds a test for
- `examples/api/lib/material/scaffold/scaffold_messenger.0.dart`
- `examples/api/lib/material/scaffold/scaffold_messenger.of.0.dart`
- `examples/api/lib/material/scaffold/scaffold_messenger.of.1.dart`

dev/bots/check_code_samples.dart

@@ -322,11 +322,8 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/search_anchor/search_anchor.1_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.2_test.dart',
   'examples/api/test/material/selection_area/selection_area.0_test.dart',
-  'examples/api/test/material/scaffold/scaffold_messenger.of.0_test.dart',
-  'examples/api/test/material/scaffold/scaffold_messenger.0_test.dart',
   'examples/api/test/material/scaffold/scaffold_state.show_bottom_sheet.0_test.dart',
   'examples/api/test/material/scaffold/scaffold_messenger_state.show_material_banner.0_test.dart',
-  'examples/api/test/material/scaffold/scaffold_messenger.of.1_test.dart',
   'examples/api/test/material/scaffold/scaffold_messenger_state.show_snack_bar.0_test.dart',
   'examples/api/test/material/app_bar/sliver_app_bar.2_test.dart',
   'examples/api/test/material/app_bar/sliver_app_bar.3_test.dart',

examples/api/test/material/scaffold/scaffold_messenger.0_test.dart

@@ -0,0 +1,22 @@
+// Copyright 2014 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 'package:flutter/material.dart';
+import 'package:flutter_api_samples/material/scaffold/scaffold_messenger.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('The snack bar should be visible after tapping the button', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ScaffoldMessengerExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'ScaffoldMessenger Sample'), findsOne);
+
+    await tester.tap(find.widgetWithText(OutlinedButton, 'Show SnackBar'));
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(SnackBar, 'A SnackBar has been shown.'), findsOne);
+  });
+}

examples/api/test/material/scaffold/scaffold_messenger.of.0_test.dart

@@ -0,0 +1,22 @@
+// Copyright 2014 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 'package:flutter/material.dart';
+import 'package:flutter_api_samples/material/scaffold/scaffold_messenger.of.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('The snack bar should be visible after tapping the button', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.OfExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'ScaffoldMessenger.of Sample'), findsOne);
+
+    await tester.tap(find.widgetWithText(ElevatedButton, 'SHOW A SNACKBAR'));
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(SnackBar, 'Have a snack!'), findsOne);
+  });
+}

examples/api/test/material/scaffold/scaffold_messenger.of.1_test.dart

@@ -0,0 +1,31 @@
+// Copyright 2014 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 'package:flutter/material.dart';
+import 'package:flutter_api_samples/material/scaffold/scaffold_messenger.of.1.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('A snack bar is displayed after 10 taps', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.OfExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'ScaffoldMessenger Demo'), findsOne);
+    expect(find.text('You have pushed the button this many times:'), findsOne);
+
+    for (int i = 0; i < 9; i++) {
+      await tester.tap(find.byType(FloatingActionButton));
+      await tester.pump();
+      expect(find.text('${i + 1}'), findsOne);
+    }
+    expect(find.byType(SnackBar), findsNothing);
+
+    await tester.tap(find.byType(FloatingActionButton));
+    await tester.pumpAndSettle();
+    expect(find.text('10'), findsOne);
+
+    expect(find.widgetWithText(SnackBar, 'A multiple of ten!'), findsOne);
+  });
+}