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/platform_selector.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+
+typedef PlatformCallback = void Function(TargetPlatform platform);
+
+class PlatformSelector extends StatefulWidget {
+  const PlatformSelector({
+    super.key,
+    required this.onChangedPlatform,
+  });
+
+  final PlatformCallback onChangedPlatform;
+
+  @override
+  State<PlatformSelector> createState() => _PlatformSelectorState();
+}
+
+class _PlatformSelectorState extends State<PlatformSelector> {
+  static const int targetPlatformStringLength = 15; // 'TargetPlatform.'.length
+
+  static String _platformToString(TargetPlatform platform) {
+    return platform.toString().substring(targetPlatformStringLength);
+  }
+
+  final TargetPlatform originaPlatform = defaultTargetPlatform;
+
+  @override
+  Widget build(BuildContext context) {
+    return SizedBox(
+      width: 170.0,
+      child: DropdownButton<TargetPlatform>(
+        value: defaultTargetPlatform,
+        icon: const Icon(Icons.arrow_downward),
+        elevation: 16,
+        onChanged: (value) {
+          if (value == null) {
+            return;
+          }
+
+          widget.onChangedPlatform(value);
+          setState(() {});
+        },
+        items: TargetPlatform.values.map((platform) {
+          return DropdownMenuItem<TargetPlatform>(
+            value: platform,
+            child: Row(
+              children: <Widget>[
+                if (platform == originaPlatform)
+                  const Icon(
+                    Icons.home,
+                    color: Color(0xff616161),
+                  ),
+                Text(_platformToString(platform)),
+              ],
+            ),
+          );
+        }).toList(),
+      ),
+    );
+  }
+}