Add @override annotation to known overriden methods

sky/sdk/example/widgets/spinning_mixed.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:sky' as sky;
 
 import 'package:sky/base/scheduler.dart' as scheduler;
 import 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/flex.dart';
 import 'package:sky/rendering/sky_binding.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/raised_button.dart';
 import 'package:sky/widgets/widget.dart';
 import 'package:vector_math/vector_math.dart';
 
 import '../rendering/solid_color_box.dart';
 
 // Solid colour, RenderObject version
 void addFlexChildSolidColor(RenderFlex parent, sky.Color backgroundColor, { int flex: 0 }) {
   RenderSolidColorBox child = new RenderSolidColorBox(backgroundColor);
   parent.add(child);
   child.parentData.flex = flex;
 }
 
 // Solid colour, Widget version
 class Rectangle extends Component {
   Rectangle(this.color, { String key }) : super(key: key);
   final Color color;
+
+  @override
   Widget build() {
     return new Flexible(
       child: new Container(
         decoration: new BoxDecoration(backgroundColor: color)
       )
     );
   }
 }
 
 Widget builder() {
@@ skipping 49 matching lines @@
   addFlexChildSolidColor(flexRoot, const sky.Color(0xFFFF00FF), flex: 1);
   flexRoot.add(proxy);
   addFlexChildSolidColor(flexRoot, const sky.Color(0xFF0000FF), flex: 1);
 
   transformBox = new RenderTransform(child: flexRoot, transform: new Matrix4.identity());
   RenderPadding root = new RenderPadding(padding: new EdgeDims.all(80.0), child: transformBox);
 
   SkyBinding.instance.root = root;
   scheduler.addPersistentFrameCallback(rotate);
 }