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
diff --git a/packages/flutter/lib/src/widgets/transitions.dart b/packages/flutter/lib/src/widgets/transitions.dart
index ac33e7b..fabeb13 100644
--- a/packages/flutter/lib/src/widgets/transitions.dart
+++ b/packages/flutter/lib/src/widgets/transitions.dart
@@ -23,6 +23,58 @@
/// [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].
///
