Add a simplified SimpleCascadingMenuApp example (flutter#149147)

The current MenuAnchor example in the API Docs is comprehensive and complicated for beginners. I have added a simple bare bone example without shortcuts, enums, etc in examples/api/lib/material/menu_anchor/ as `menu_anchor.3.dart`. The example is contributed by @mafreud

Fixes flutter#148104

examples/api/lib/material/menu_anchor/menu_anchor.3.dart

@@ -0,0 +1,81 @@
+// 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';
+
+/// Flutter code sample for [SimpleCascadingMenuApp].
+
+void main() => runApp(const SimpleCascadingMenuApp());
+
+/// A Simple Cascading Menu example using the [MenuAnchor] Widget.
+class MyCascadingMenu extends StatefulWidget {
+  const MyCascadingMenu({super.key});
+
+  @override
+  State<MyCascadingMenu> createState() => _MyCascadingMenuState();
+}
+
+class _MyCascadingMenuState extends State<MyCascadingMenu> {
+  final FocusNode _buttonFocusNode = FocusNode(debugLabel: 'Menu Button');
+
+  @override
+  void dispose() {
+    _buttonFocusNode.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return MenuAnchor(
+      childFocusNode: _buttonFocusNode,
+      menuChildren: <Widget>[
+        MenuItemButton(
+          onPressed: () {},
+          child: const Text('Revert'),
+        ),
+        MenuItemButton(
+          onPressed: () {},
+          child: const Text('Setting'),
+        ),
+        MenuItemButton(
+          onPressed: () {},
+          child: const Text('Send Feedback'),
+        ),
+      ],
+      builder: (_, MenuController controller, Widget? child) {
+        return IconButton(
+          focusNode: _buttonFocusNode,
+          onPressed: () {
+            if (controller.isOpen) {
+              controller.close();
+            } else {
+              controller.open();
+            }
+          },
+          icon: const Icon(Icons.more_vert),
+        );
+      },
+    );
+  }
+}
+
+/// Top Level Application Widget.
+class SimpleCascadingMenuApp extends StatelessWidget {
+  const SimpleCascadingMenuApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      debugShowCheckedModeBanner: false,
+      home: Scaffold(
+        appBar: AppBar(
+          title: const Text('MenuAnchor Simple Example'),
+          actions: const <Widget>[
+            MyCascadingMenu(),
+          ],
+        ),
+      ),
+    );
+  }
+}

examples/api/test/material/menu_anchor/menu_anchor.3_test.dart

@@ -0,0 +1,36 @@
+// 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/menu_anchor/menu_anchor.3.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+ testWidgets('Menu button opens and closes the menu', (WidgetTester tester) async {
+   await tester.pumpWidget(const example.SimpleCascadingMenuApp());
+
+   // Find the menu button.
+   final Finder menuButton = find.byType(IconButton);
+   expect(menuButton, findsOneWidget);
+
+   // Tap the menu button to open the menu.
+   await tester.tap(menuButton);
+   await tester.pumpAndSettle();
+
+   // Verify that the menu is open.
+   expect(find.text('Revert'), findsOneWidget);
+
+   // Tap the menu button again to close the menu.
+   await tester.tap(menuButton);
+   await tester.pumpAndSettle();
+
+   // Verify that the menu is closed.
+   expect(find.text('Revert'), findsNothing);
+ });
+
+ testWidgets('Does not show debug banner', (WidgetTester tester) async {
+   await tester.pumpWidget(const example.SimpleCascadingMenuApp());
+   expect(find.byType(CheckedModeBanner), findsNothing);
+ });
+}

packages/flutter/lib/src/material/menu_anchor.dart

@@ -119,6 +119,13 @@ typedef MenuAnchorChildBuilder = Widget Function(
 ///
 /// ** See code in examples/api/lib/material/menu_anchor/menu_anchor.1.dart **
 /// {@end-tool}
+///
+/// {@tool dartpad}
+/// This example demonstrates a simplified cascading menu using the [MenuAnchor]
+/// widget.
+///
+/// ** See code in examples/api/lib/material/menu_anchor/menu_anchor.3.dart **
+/// {@end-tool}
 class MenuAnchor extends StatefulWidget {
   /// Creates a const [MenuAnchor].
   ///