Add sample code to RelativePositionedTransition (#63940)

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

@@ -924,6 +924,61 @@ class PositionedTransition extends AnimatedWidget {
 /// animated by a [CurvedAnimation] set to [Curves.elasticInOut]:
 /// {@animation 300 378 https://flutter.github.io/assets-for-api-docs/assets/widgets/relative_positioned_transition.mp4}
 ///
+/// {@tool dartpad --template=stateful_widget_material_ticker}
+///
+/// The following code implements the [RelativePositionedTransition] as seen in the video
+/// above:
+///
+/// ```dart
+/// AnimationController _controller;
+///
+/// @override
+/// void initState() {
+///   super.initState();
+///   _controller = AnimationController(
+///     duration: const Duration(seconds: 2),
+///     vsync: this,
+///   )..repeat(reverse: true);
+/// }
+///
+/// @override
+/// void dispose() {
+///   _controller.dispose();
+///   super.dispose();
+/// }
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   final double smallLogo = 100;
+///   final double bigLogo = 200;
+///
+///   return LayoutBuilder(
+///     builder: (context, constraints) {
+///       final Size biggest = constraints.biggest;
+///       return Stack(
+///         children: [
+///           RelativePositionedTransition(
+///             size: biggest,
+///             rect: RectTween(
+///               begin: Rect.fromLTWH(0, 0, bigLogo, bigLogo),
+///               end: Rect.fromLTWH(biggest.width - smallLogo, biggest.height - smallLogo, smallLogo, smallLogo),
+///             ).animate(CurvedAnimation(
+///               parent: _controller,
+///               curve: Curves.elasticInOut,
+///             )),
+///             child: Padding(
+///               padding: const EdgeInsets.all(8),
+///               child: FlutterLogo()
+///             ),
+///           ),
+///         ],
+///       );
+///     },
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [PositionedTransition], a widget that animates its child from a start