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/image_page_test.dart

@@ -0,0 +1,47 @@
+import 'package:context_menus/image_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(
+    'Gives correct behavior for all values of contextMenuBuilder',
+    (tester) async {
+      await tester.pumpWidget(const MyApp());
+
+      // Navigate to the ImagePage example.
+      await tester.dragUntilVisible(
+        find.text(ImagePage.title),
+        find.byType(ListView),
+        const Offset(0.0, -100.0),
+      );
+      await tester.pumpAndSettle();
+      await tester.tap(find.text(ImagePage.title));
+      await tester.pumpAndSettle();
+      expect(
+        find.descendant(
+          of: find.byType(AppBar),
+          matching: find.text(ImagePage.title),
+        ),
+        findsOneWidget,
+      );
+
+      // Right click on the FlutterLogo.
+      TestGesture gesture = await tester.startGesture(
+        tester.getCenter(find.byType(FlutterLogo)),
+        kind: PointerDeviceKind.mouse,
+        buttons: kSecondaryMouseButton,
+      );
+      await tester.pump();
+      await gesture.up();
+      await gesture.removePointer();
+      await tester.pumpAndSettle();
+
+      // The default context menu is shown with a custom button.
+      expect(find.byType(AdaptiveTextSelectionToolbar), findsOneWidget);
+      expect(find.text('Save'), findsOneWidget);
+    },
+    variant: TargetPlatformVariant.all(),
+  );
+}