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 `///`).

isolate_example/lib/infinite_process_page.dart

@@ -78,25 +78,25 @@ class InfiniteProcessPage extends StatelessWidget {
                     value: !controller.paused,
                     onChanged: (_) => controller.pausedSwitch(),
                     activeTrackColor: Colors.lightGreenAccent,
-                    activeColor: Colors.black,
+                    activeThumbColor: Colors.black,
                     inactiveTrackColor: Colors.deepOrangeAccent,
                     inactiveThumbColor: Colors.black,
                   ),
                   const Text('Pause/Resume'),
                 ],
               ),
-              Row(
-                mainAxisAlignment: MainAxisAlignment.center,
-                children: [
-                  for (int i = 1; i <= 3; i++) ...[
-                    Radio<int>(
-                      value: i,
-                      groupValue: controller.currentMultiplier,
-                      onChanged: (val) => controller.setMultiplier(val!),
-                    ),
-                    Text('${i}x'),
+              RadioGroup(
+                groupValue: controller.currentMultiplier,
+                onChanged: (val) => controller.setMultiplier(val!),
+                child: Row(
+                  mainAxisAlignment: MainAxisAlignment.center,
+                  children: [
+                    for (int i = 1; i <= 3; i++) ...[
+                      Radio<int>(value: i),
+                      Text('${i}x'),
+                    ],
                   ],
-                ],
+                ),
               ),
             ],
           ),