Dispose AnimationController in examples (#5207)

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

Fixes #5206

examples/layers/services/isolate.dart

@@ -227,6 +227,12 @@ class IsolateExampleState extends State<StatefulWidget> {
     );
   }
 
+  @override
+  void dispose() {
+    _animation.dispose();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
     return new Material(

examples/layers/widgets/spinning_square.dart

@@ -23,6 +23,12 @@ class _SpinningSquareState extends State<SpinningSquare> {
     )..repeat();
   }
 
+  @override
+  void dispose() {
+    _animation.dispose();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
     return new RotationTransition(