Dispose AnimationController in examples (#5207)

These examples should show the best practices for working with
AnimationControllers.

Fixes #5206
diff --git a/examples/layers/widgets/spinning_square.dart b/examples/layers/widgets/spinning_square.dart
index 8af6deb..155ce61 100644
--- a/examples/layers/widgets/spinning_square.dart
+++ b/examples/layers/widgets/spinning_square.dart
@@ -24,6 +24,12 @@
   }
 
   @override
+  void dispose() {
+    _animation.dispose();
+    super.dispose();
+  }
+
+  @override
   Widget build(BuildContext context) {
     return new RotationTransition(
       turns: _animation,