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/default_values_page_test.dart

@@ -0,0 +1,99 @@
+import 'package:context_menus/default_values_page.dart';
+import 'package:context_menus/main.dart';
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.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 DefaultValuesPage example.
+    await tester.dragUntilVisible(
+      find.text(DefaultValuesPage.title),
+      find.byType(ListView),
+      const Offset(0.0, -100.0),
+    );
+    await tester.pumpAndSettle();
+    await tester.tap(find.text(DefaultValuesPage.title));
+    await tester.pumpAndSettle();
+    expect(
+      find.descendant(
+        of: find.byType(AppBar),
+        matching: find.text(DefaultValuesPage.title),
+      ),
+      findsOneWidget,
+    );
+
+    // Right click on the text field where contextMenuBuilder isn't passed.
+    TestGesture gesture = await tester.startGesture(
+      tester.getCenter(find.byType(EditableText).first),
+      kind: PointerDeviceKind.mouse,
+      buttons: kSecondaryMouseButton,
+    );
+    await tester.pump();
+    await gesture.up();
+    await gesture.removePointer();
+    await tester.pumpAndSettle();
+
+    // The default context menu is shown.
+    expect(find.byType(AdaptiveTextSelectionToolbar), findsOneWidget);
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.iOS:
+        expect(
+            find.byType(CupertinoTextSelectionToolbarButton), findsNWidgets(2));
+      case TargetPlatform.macOS:
+        expect(find.byType(CupertinoDesktopTextSelectionToolbarButton),
+            findsNWidgets(2));
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+        expect(find.byType(TextSelectionToolbarTextButton), findsNWidgets(1));
+      case TargetPlatform.linux:
+      case TargetPlatform.windows:
+        expect(
+            find.byType(DesktopTextSelectionToolbarButton), findsNWidgets(1));
+    }
+
+    // Tap the next field to hide the context menu.
+    await tester.tap(find.byType(EditableText).at(1));
+    await tester.pumpAndSettle();
+    expect(find.byType(AdaptiveTextSelectionToolbar), findsNothing);
+
+    // Right click on the text field where contextMenuBuilder is given null.
+    gesture = await tester.startGesture(
+      tester.getCenter(find.byType(EditableText).at(1)),
+      kind: PointerDeviceKind.mouse,
+      buttons: kSecondaryMouseButton,
+    );
+    await tester.pump();
+    await gesture.up();
+    await gesture.removePointer();
+    await tester.pumpAndSettle();
+
+    // No context menu is shown.
+    expect(find.byType(AdaptiveTextSelectionToolbar), findsNothing);
+
+    // Tap the next field to hide the context menu.
+    await tester.tap(find.byType(EditableText).at(2));
+    await tester.pumpAndSettle();
+    expect(find.byType(AdaptiveTextSelectionToolbar), findsNothing);
+
+    // Right click on the text field with the custom contextMenuBuilder.
+    gesture = await tester.startGesture(
+      tester.getCenter(find.byType(EditableText).at(2)),
+      kind: PointerDeviceKind.mouse,
+      buttons: kSecondaryMouseButton,
+    );
+    await tester.pump();
+    await gesture.up();
+    await gesture.removePointer();
+    await tester.pumpAndSettle();
+
+    // The custom context menu is shown.
+    expect(find.byType(AdaptiveTextSelectionToolbar), findsOneWidget);
+    expect(find.text('Custom button'), findsOneWidget);
+  });
+}