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/anywhere_page_test.dart

@@ -0,0 +1,36 @@
+import 'package:context_menus/anywhere_page.dart';
+import 'package:context_menus/main.dart';
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Right click works outside of text', (tester) async {
+    await tester.pumpWidget(const MyApp());
+
+    // Navigate to the AnywherePage example.
+    await tester.dragUntilVisible(
+      find.text(AnywherePage.title),
+      find.byType(ListView),
+      const Offset(0.0, -100.0),
+    );
+    await tester.pumpAndSettle();
+    await tester.tap(find.text(AnywherePage.title));
+    await tester.pumpAndSettle();
+
+    // Right click on the background of the app to show the context menu.
+    final TestGesture gesture = await tester.startGesture(
+      const Offset(100.0, 100.0),
+      kind: PointerDeviceKind.mouse,
+      buttons: kSecondaryMouseButton,
+    );
+    await tester.pump();
+    await gesture.up();
+    await gesture.removePointer();
+    await tester.pumpAndSettle();
+
+    // The context menu is shown, with a custom back button.
+    expect(find.byType(AdaptiveTextSelectionToolbar), findsOneWidget);
+    expect(find.text('Back'), findsOneWidget);
+  });
+}