Add test for platform_menu_bar.0.dart (#157328)

Contributes to https://github.com/flutter/flutter/issues/130459

It adds a test for
- `examples/api/test/material/platform_menu_bar/platform_menu_bar.0_test.dart`

dev/bots/check_code_samples.dart

@@ -310,7 +310,6 @@ class SampleChecker {
 // See https://github.com/flutter/flutter/issues/130459
 final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/color_scheme/dynamic_content_color.0_test.dart',
-  'examples/api/test/material/platform_menu_bar/platform_menu_bar.0_test.dart',
   'examples/api/test/painting/star_border/star_border.0_test.dart',
   'examples/api/test/widgets/navigator/navigator.restorable_push_and_remove_until.0_test.dart',
   'examples/api/test/widgets/navigator/navigator.restorable_push.0_test.dart',

examples/api/test/material/platform_menu_bar/platform_menu_bar.0_test.dart

@@ -0,0 +1,124 @@
+// 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/services.dart';
+import 'package:flutter_api_samples/material/platform_menu_bar/platform_menu_bar.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  late _FakeMenuChannel fakeMenuChannel;
+  late PlatformMenuDelegate originalDelegate;
+  late DefaultPlatformMenuDelegate delegate;
+
+  setUp(() {
+    fakeMenuChannel = _FakeMenuChannel((MethodCall call) async {});
+    delegate = DefaultPlatformMenuDelegate(channel: fakeMenuChannel);
+    originalDelegate = WidgetsBinding.instance.platformMenuDelegate;
+    WidgetsBinding.instance.platformMenuDelegate = delegate;
+  });
+
+  tearDown(() {
+    WidgetsBinding.instance.platformMenuDelegate = originalDelegate;
+  });
+
+  testWidgets('PlatformMenuBar creates a menu', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.ExampleApp(),
+    );
+
+    expect(
+      find.text('This space intentionally left blank.\nShow a message here using the menu.'),
+      findsOne,
+    );
+    expect(find.byType(PlatformMenuBar), findsOne);
+
+    expect(
+      fakeMenuChannel.outgoingCalls.last.method,
+      'Menu.setMenus',
+    );
+    expect(
+      fakeMenuChannel.outgoingCalls.last.arguments,
+      equals(const <String, Object?>{
+        '0': <Map<String, Object>>[
+          <String, Object>{
+            'id': 11,
+            'label': 'Flutter API Sample',
+            'enabled': true,
+            'children': <Map<String, Object>>[
+              <String, Object>{'id': 2, 'label': 'About', 'enabled': true},
+              <String, Object>{'id': 3, 'isDivider': true},
+              <String, Object>{
+                'id': 5,
+                'label': 'Show Message',
+                'enabled': true,
+                'shortcutCharacter': 'm',
+                'shortcutModifiers': 0,
+              },
+              <String, Object>{
+                'id': 8,
+                'label': 'Messages',
+                'enabled': true,
+                'children': <Map<String, Object>>[
+                  <String, Object>{
+                    'id': 6,
+                    'label': 'I am not throwing away my shot.',
+                    'enabled': true,
+                    'shortcutTrigger': 49,
+                    'shortcutModifiers': 1,
+                  },
+                  <String, Object>{
+                    'id': 7,
+                    'label': "There's a million things I haven't done, but just you wait.",
+                    'enabled': true,
+                    'shortcutTrigger': 50,
+                    'shortcutModifiers': 1,
+                  },
+                ],
+              },
+              <String, Object>{'id': 9, 'isDivider': true},
+              <String, Object>{'id': 10, 'enabled': true, 'platformProvidedMenu': 1},
+            ],
+          },
+        ],
+      }),
+    );
+  }, variant: const TargetPlatformVariant(<TargetPlatform>{TargetPlatform.macOS}));
+}
+
+class _FakeMenuChannel implements MethodChannel {
+  _FakeMenuChannel(this.outgoing);
+
+  Future<dynamic> Function(MethodCall) outgoing;
+  Future<void> Function(MethodCall)? incoming;
+
+  List<MethodCall> outgoingCalls = <MethodCall>[];
+
+  @override
+  BinaryMessenger get binaryMessenger => throw UnimplementedError();
+
+  @override
+  MethodCodec get codec => const StandardMethodCodec();
+
+  @override
+  Future<List<T>> invokeListMethod<T>(String method, [dynamic arguments]) => throw UnimplementedError();
+
+  @override
+  Future<Map<K, V>> invokeMapMethod<K, V>(String method, [dynamic arguments]) => throw UnimplementedError();
+
+  @override
+  Future<T> invokeMethod<T>(String method, [dynamic arguments]) async {
+    final MethodCall call = MethodCall(method, arguments);
+    outgoingCalls.add(call);
+    return await outgoing(call) as T;
+  }
+
+  @override
+  String get name => 'flutter/menu';
+
+  @override
+  void setMethodCallHandler(Future<void> Function(MethodCall call)? handler) => incoming = handler;
+}