Make AnimatedSwitcher example into a dartpad example (#52547)

packages/flutter/lib/src/widgets/animated_switcher.dart

@@ -87,23 +87,20 @@ typedef AnimatedSwitcherLayoutBuilder = Widget Function(Widget currentChild, Lis
 /// progress indicator and the image will be fading out while a new progress
 /// indicator is fading in.)
 ///
-/// {@tool snippet}
+/// The type of transition can be changed from a cross-fade to a custom
+/// transition by setting the [transitionBuilder].
 ///
-/// ```dart
-/// class ClickCounter extends StatefulWidget {
-///   const ClickCounter({Key key}) : super(key: key);
-///
-///   @override
-///   _ClickCounterState createState() => _ClickCounterState();
-/// }
+/// {@tool dartpad --template=stateful_widget_material}
+/// This sample shows a counter that animates the scale of a text widget
+/// whenever the value changes.
 ///
-/// class _ClickCounterState extends State<ClickCounter> {
+/// ```dart
 /// int _count = 0;
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///     return MaterialApp(
-///       home: Material(
+///   return Container(
+///     color: Colors.white,
 ///     child: Column(
 ///       mainAxisAlignment: MainAxisAlignment.center,
 ///       children: <Widget>[
@@ -131,10 +128,8 @@ typedef AnimatedSwitcherLayoutBuilder = Widget Function(Widget currentChild, Lis
 ///         ),
 ///       ],
 ///     ),
-///       ),
 ///   );
 /// }
-/// }
 /// ```
 /// {@end-tool}
 ///