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/lib/modified_action_page.dart

@@ -0,0 +1,97 @@
+import 'package:flutter/material.dart';
+import 'package:url_launcher/url_launcher.dart';
+
+import 'constants.dart';
+import 'platform_selector.dart';
+
+class ModifiedActionPage extends StatelessWidget {
+  ModifiedActionPage({
+    super.key,
+    required this.onChangedPlatform,
+  });
+
+  static const String route = 'modified-action';
+  static const String title = 'Modified Action';
+  static const String subtitle =
+      'The copy button copies but also shows a menu.';
+  static const String url = '$kCodeUrl/modified_action_page.dart';
+
+  final PlatformCallback onChangedPlatform;
+
+  final TextEditingController _controller = TextEditingController(
+    text: 'Try using the copy button.',
+  );
+
+  DialogRoute _showDialog(BuildContext context) {
+    return DialogRoute<void>(
+      context: context,
+      builder: (context) => const AlertDialog(
+          title: Text('Copied, but also showed this dialog.')),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text(ModifiedActionPage.title),
+        actions: <Widget>[
+          PlatformSelector(
+            onChangedPlatform: onChangedPlatform,
+          ),
+          IconButton(
+            icon: const Icon(Icons.code),
+            onPressed: () async {
+              if (!await launchUrl(Uri.parse(url))) {
+                throw 'Could not launch $url';
+              }
+            },
+          ),
+        ],
+      ),
+      body: Center(
+        child: SizedBox(
+          width: 300.0,
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.center,
+            children: <Widget>[
+              const Text(
+                'This example shows adding to the behavior of a default button.',
+              ),
+              const SizedBox(
+                height: 30.0,
+              ),
+              TextField(
+                controller: _controller,
+                contextMenuBuilder: (context, editableTextState) {
+                  final List<ContextMenuButtonItem> buttonItems =
+                      editableTextState.contextMenuButtonItems;
+                  // Modify the copy buttonItem to show a dialog after copying.
+                  final int copyButtonIndex = buttonItems.indexWhere(
+                    (buttonItem) {
+                      return buttonItem.type == ContextMenuButtonType.copy;
+                    },
+                  );
+                  if (copyButtonIndex >= 0) {
+                    final ContextMenuButtonItem copyButtonItem =
+                        buttonItems[copyButtonIndex];
+                    buttonItems[copyButtonIndex] = copyButtonItem.copyWith(
+                      onPressed: () {
+                        copyButtonItem.onPressed!();
+                        Navigator.of(context).push(_showDialog(context));
+                      },
+                    );
+                  }
+                  return AdaptiveTextSelectionToolbar.buttonItems(
+                    anchors: editableTextState.contextMenuAnchors,
+                    buttonItems: buttonItems,
+                  );
+                },
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}