Move experimental (#2680)

Removes the remaining samples in `experimental`. They're now in the
flutter/demos repository.

*List which issues are fixed by this PR. For larger changes, raising an
issue first helps
reduce redundant work.*

#2520 

## 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 have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

material_3_demo/lib/src/component_screen.dart

@@ -981,35 +981,30 @@ class _RadiosState extends State<Radios> {
     return ComponentDecoration(
       label: 'Radio buttons',
       tooltipMessage: 'Use RadioListTile<T> or Radio<T>',
-      child: Column(
-        children: <Widget>[
-          RadioListTile<Options>(
-            title: const Text('Option 1'),
-            value: Options.option1,
-            groupValue: _selectedOption,
-            onChanged: (value) {
-              setState(() {
-                _selectedOption = value;
-              });
-            },
-          ),
-          RadioListTile<Options>(
-            title: const Text('Option 2'),
-            value: Options.option2,
-            groupValue: _selectedOption,
-            onChanged: (value) {
-              setState(() {
-                _selectedOption = value;
-              });
-            },
-          ),
-          RadioListTile<Options>(
-            title: const Text('Option 3'),
-            value: Options.option3,
-            groupValue: _selectedOption,
-            onChanged: null,
-          ),
-        ],
+      child: RadioGroup(
+        groupValue: _selectedOption,
+        onChanged: (value) {
+          setState(() {
+            _selectedOption = value;
+          });
+        },
+        child: Column(
+          children: <Widget>[
+            RadioListTile<Options>(
+              title: const Text('Option 1'),
+              value: Options.option1,
+            ),
+            RadioListTile<Options>(
+              title: const Text('Option 2'),
+              value: Options.option2,
+            ),
+            RadioListTile<Options>(
+              title: const Text('Option 3'),
+              value: Options.option3,
+              enabled: false,
+            ),
+          ],
+        ),
       ),
     );
   }