Add sample code to DefaultTextStyleTransition (#64638)

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

@@ -1310,6 +1310,51 @@ class AlignTransition extends AnimatedWidget {
 /// Animated version of a [DefaultTextStyle] that animates the different properties
 /// of its [TextStyle].
 ///
+/// {@tool dartpad --template=stateful_widget_material_ticker}
+///
+/// The following code implements the [DefaultTextStyleTransition] that shows
+/// a transition between thick blue font and thin red font.
+///
+/// ```dart
+/// AnimationController _controller;
+/// TextStyleTween _styleTween;
+/// CurvedAnimation _curvedAnimation;
+///
+/// @override
+/// void initState() {
+///   super.initState();
+///   _controller = AnimationController(
+///     duration: const Duration(seconds: 2),
+///     vsync: this,
+///   )..repeat(reverse: true);
+///   _styleTween = TextStyleTween(
+///     begin: TextStyle(fontSize: 50, color: Colors.blue, fontWeight: FontWeight.w900),
+///     end: TextStyle(fontSize: 50, color: Colors.red, fontWeight: FontWeight.w100),
+///   );
+///   _curvedAnimation = CurvedAnimation(
+///     parent: _controller,
+///     curve: Curves.elasticInOut,
+///   );
+/// }
+///
+/// @override
+/// void dispose() {
+///   _controller.dispose();
+///   super.dispose();
+/// }
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return Center(
+///     child: DefaultTextStyleTransition(
+///       style: _styleTween.animate(_curvedAnimation),
+///       child: Text('Flutter'),
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [AnimatedDefaultTextStyle], which animates changes in text style without