Add tests for scaffold messenger state (flutter#152735)

Contributes to flutter#130459

It adds a test for
- `examples/api/lib/material/scaffold/scaffold_messenger_state.show_snack_bar.0.dart`
- `examples/api/lib/material/scaffold/scaffold_messenger_state.show_material_banner.0.dart`

Author: ValentinVignal

dev/bots/check_code_samples.dart

@@ -320,8 +320,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/platform_menu_bar/platform_menu_bar.0_test.dart',
   'examples/api/test/material/flexible_space_bar/flexible_space_bar.0_test.dart',
   'examples/api/test/material/selection_area/selection_area.0_test.dart',
-  'examples/api/test/material/scaffold/scaffold_messenger_state.show_material_banner.0_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/navigation_rail/navigation_rail.extended_animation.0_test.dart',

examples/api/test/material/scaffold/scaffold_messenger_state.show_material_banner.0_test.dart

@@ -0,0 +1,25 @@
+// 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_state.show_material_banner.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Pressing the button should show a material banner', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ShowMaterialBannerExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'ScaffoldMessengerState Sample'), findsOne);
+    await tester.tap(find.widgetWithText(OutlinedButton, 'Show MaterialBanner'));
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(MaterialBanner, 'This is a MaterialBanner'), findsOne);
+    expect(
+      find.descendant(of: find.byType(MaterialBanner), matching: find.widgetWithText(TextButton, 'DISMISS')),
+      findsOne,
+    );
+  });
+}

examples/api/test/material/scaffold/scaffold_messenger_state.show_snack_bar.0_test.dart

@@ -0,0 +1,21 @@
+// 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_state.show_snack_bar.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Pressing the button should display a snack bar', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ShowSnackBarExampleApp(),
+    );
+
+    expect(find.widgetWithText(AppBar, 'ScaffoldMessengerState Sample'), findsOne);
+    await tester.tap(find.widgetWithText(OutlinedButton, 'Show SnackBar'));
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(SnackBar, 'A SnackBar has been shown.'), findsOne);
+  });
+}