Add sample for ValueListenableBuilder (#30626)

* Add sample for ValueListenableBuilder

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

@@ -38,6 +38,65 @@ typedef ValueWidgetBuilder<T> = Widget Function(BuildContext context, T value, W
 /// Using this pre-built child is entirely optional, but can improve
 /// performance significantly in some cases and is therefore a good practice.
 ///
+/// {@tool sample}
+///
+/// This sample shows how you could use a [ValueListenableBuilder] instead of
+/// setting state on the whole [Scaffold] in the default `flutter create` app.
+///
+/// ```dart
+/// class MyHomePage extends StatefulWidget {
+///   MyHomePage({Key key, this.title}) : super(key: key);
+///   final String title;
+///
+///   @override
+///   _MyHomePageState createState() => _MyHomePageState();
+/// }
+///
+/// class _MyHomePageState extends State<MyHomePage> {
+///   final ValueNotifier<int> _counter = ValueNotifier<int>(0);
+///   final Widget goodJob = const Text('Good job!');
+///   @override
+///   Widget build(BuildContext context) {
+///     return Scaffold(
+///       appBar: AppBar(
+///         title: Text(widget.title)
+///       ),
+///       body: Center(
+///         child: Column(
+///           mainAxisAlignment: MainAxisAlignment.center,
+///           children: <Widget>[
+///             Text('You have pushed the button this many times:'),
+///             ValueListenableBuilder(
+///               builder: (BuildContext context, int value, Widget child) {
+///                 // This builder will only get called when the _counter
+///                 // is updated.
+///                 return Row(
+///                   mainAxisAlignment: MainAxisAlignment.spaceEvenly,
+///                   children: <Widget>[
+///                     Text('$value'),
+///                     child,
+///                   ],
+///                 );
+///               },
+///               valueListenable: _counter,
+///               // The child parameter is most helpful if the child is
+///               // expensive to build and does not depend on the value from
+///               // the notifier.
+///               child: goodJob,
+///             )
+///           ],
+///         ),
+///       ),
+///       floatingActionButton: FloatingActionButton(
+///         child: Icon(Icons.plus_one),
+///         onPressed: () => _counter.value += 1,
+///       ),
+///     );
+///   }
+/// }
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [AnimatedBuilder], which also triggers rebuilds from a [Listenable]