Move context menus out of experimental (#2134)

Now that the context menu code is on stable, move the sample out of
experimental.

Fixes https://github.com/flutter/samples/issues/2110

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

context_menus/test/cascading_menu_page_test.dart

@@ -0,0 +1,126 @@
+import 'package:context_menus/cascading_menu_page.dart';
+import 'package:context_menus/main.dart';
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Can show and use the cascading menu', (tester) async {
+    await tester.pumpWidget(const MyApp());
+
+    // Navigate to the CascadingMenuPage example.
+    await tester.dragUntilVisible(
+      find.text(CascadingMenuPage.title),
+      find.byType(ListView),
+      const Offset(0.0, -250.0),
+    );
+    await tester.pumpAndSettle();
+    await tester.tap(find.text(CascadingMenuPage.title));
+    await tester.pumpAndSettle();
+
+    const String message =
+        'Right click or long press anywhere to show the cascading menu.';
+    expect(find.text(message), findsOneWidget);
+
+    // Right click on the background of the app to show the cascading context
+    // menu.
+    TestGesture gesture = await tester.startGesture(
+      const Offset(0.0, 100.0),
+      kind: PointerDeviceKind.mouse,
+      buttons: kSecondaryMouseButton,
+    );
+    await tester.pump();
+    await gesture.up();
+    await gesture.removePointer();
+    await tester.pumpAndSettle();
+
+    // The custom cascading context menu is shown.
+    expect(find.byType(DesktopTextSelectionToolbar), findsOneWidget);
+    expect(find.text('About'), findsOneWidget);
+    expect(find.text('Show'), findsNothing);
+    expect(find.text('Hide'), findsOneWidget);
+    expect(find.text('Reset'), findsOneWidget);
+    expect(find.text('Color'), findsOneWidget);
+
+    // Click "Hide Message" and the message and menu are hidden.
+    await tester.tap(find.text('Hide'));
+    await tester.pumpAndSettle();
+    expect(find.byType(DesktopTextSelectionToolbar), findsNothing);
+    expect(find.text(message), findsNothing);
+
+    // Right click to show the menu again.
+    gesture = await tester.startGesture(
+      const Offset(0.0, 100.0),
+      kind: PointerDeviceKind.mouse,
+      buttons: kSecondaryMouseButton,
+    );
+    await tester.pump();
+    await gesture.up();
+    await gesture.removePointer();
+    await tester.pumpAndSettle();
+    expect(find.byType(DesktopTextSelectionToolbar), findsOneWidget);
+    expect(find.text('About'), findsOneWidget);
+    expect(find.text('Show'), findsOneWidget);
+    expect(find.text('Hide'), findsNothing);
+    expect(find.text('Reset'), findsOneWidget);
+    expect(find.text('Color'), findsOneWidget);
+
+    // Use the shortcut to show the message again.
+    await tester.sendKeyDownEvent(LogicalKeyboardKey.control);
+    await tester.sendKeyDownEvent(LogicalKeyboardKey.keyS);
+    await tester.sendKeyUpEvent(LogicalKeyboardKey.control);
+    await tester.sendKeyUpEvent(LogicalKeyboardKey.keyS);
+    await tester.pumpAndSettle();
+    expect(find.byType(DesktopTextSelectionToolbar), findsNothing);
+    expect(find.text(message), findsOneWidget);
+
+    // Right click to show the menu again.
+    gesture = await tester.startGesture(
+      const Offset(0.0, 100.0),
+      kind: PointerDeviceKind.mouse,
+      buttons: kSecondaryMouseButton,
+    );
+    await tester.pump();
+    await gesture.up();
+    await gesture.removePointer();
+    await tester.pumpAndSettle();
+    expect(find.byType(DesktopTextSelectionToolbar), findsOneWidget);
+    expect(find.text('About'), findsOneWidget);
+    expect(find.text('Show'), findsNothing);
+    expect(find.text('Hide'), findsOneWidget);
+    expect(find.text('Reset'), findsOneWidget);
+    expect(find.text('Color'), findsOneWidget);
+    expect(find.text('Red'), findsNothing);
+    expect(find.text('Green'), findsNothing);
+    expect(find.text('Blue'), findsNothing);
+
+    // Hover Background Color.
+    gesture = await tester.startGesture(
+      //tester.getCenter(find.text('Color')),
+      const Offset(0.0, 100),
+      kind: PointerDeviceKind.mouse,
+    );
+    await tester.pump();
+    await gesture.moveTo(tester.getCenter(find.text('Color')));
+    await tester.pumpAndSettle();
+
+    // Background colors are shown.
+    expect(find.text('Red'), findsOneWidget);
+    expect(find.text('Green'), findsOneWidget);
+    expect(find.text('Blue'), findsOneWidget);
+
+    // Select green.
+    await gesture.moveTo(tester.getCenter(find.text('Green')));
+    await tester.pump();
+    await tester.tap(find.text('Green'));
+    await tester.pump();
+    await gesture.removePointer();
+
+    // The menu is gone and the UI shows green was selected.
+    expect(find.byType(DesktopTextSelectionToolbar), findsNothing);
+    expect(find.text('Last Selected: Green Background'), findsOneWidget);
+  },
+      // TODO(justinmc): https://github.com/flutter/samples/issues/2086
+      skip: true);
+}