Fix-up code sample for TweenSequence (#34679)

packages/flutter/lib/src/animation/tween_sequence.dart

@@ -7,6 +7,9 @@ import 'package:flutter/foundation.dart';
 import 'animation.dart';
 import 'tween.dart';
 
+// Examples can assume:
+// AnimationController myAnimationController;
+
 /// Enables creating an [Animation] whose value is defined by a sequence of
 /// [Tween]s.
 ///
@@ -14,9 +17,10 @@ import 'tween.dart';
 /// animation's duration. Each tween defines the animation's value during the
 /// interval indicated by its weight.
 ///
-/// For example, to define an animation that uses an easing curve to interpolate
-/// between 5.0 and 10.0 during the first 40% of the animation, remain at 10.0
-/// for the next 20%, and then return to 10.0 for the final 40%:
+/// {@tool sample}
+/// This example defines an animation that uses an easing curve to interpolate
+/// between 5.0 and 10.0 during the first 40% of the animation, remains at 10.0
+/// for the next 20%, and then returns to 5.0 for the final 40%.
 ///
 /// ```dart
 /// final Animation<double> animation = TweenSequence(
@@ -38,6 +42,7 @@ import 'tween.dart';
 ///   ],
 /// ).animate(myAnimationController);
 /// ```
+/// {@end-tool}
 class TweenSequence<T> extends Animatable<T> {
   /// Construct a TweenSequence.
   ///