Added sample code to AnimatedWidget (#27803)

* Added sample to AnimatedWidget

* Updated AnimatedWidget example to use listenable from parent

* Made progress private and changed widget name

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

@@ -23,6 +23,58 @@ export 'package:flutter/rendering.dart' show RelativeRect;
 /// [AnimatedWidget] is most useful for widgets that are otherwise stateless. To
 /// use [AnimatedWidget], simply subclass it and implement the build function.
 ///
+///{@tool sample}
+///
+/// This code defines a widget called `Spinner` that spins a green square
+/// continually. It is built with an [AnimatedWidget].
+///
+/// ```dart
+/// class Spinner extends StatefulWidget {
+///   @override
+///   _SpinnerState createState() => _SpinnerState();
+/// }
+///
+/// class _SpinnerState extends State<Spinner> with TickerProviderStateMixin {
+///   AnimationController _controller;
+///
+///   @override
+///   void initState() {
+///     super.initState();
+///     _controller = AnimationController(
+///       duration: const Duration(seconds: 10),
+///       vsync: this,
+///     )..repeat();
+///   }
+///
+///   @override
+///   void dispose() {
+///     _controller.dispose();
+///     super.dispose();
+///   }
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return SpinningContainer(controller: _controller);
+///   }
+/// }
+///
+/// class SpinningContainer extends AnimatedWidget {
+///   const SpinningContainer({Key key, AnimationController controller})
+///       : super(key: key, listenable: controller);
+///
+///   Animation<double> get _progress => listenable;
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return Transform.rotate(
+///       angle: _progress.value * 2.0 * math.pi,
+///       child: Container(width: 200.0, height: 200.0, color: Colors.green),
+///     );
+///   }
+/// }
+/// ```
+/// {@end-tool}
+///
 /// For more complex case involving additional state, consider using
 /// [AnimatedBuilder].
 ///