Add @override annotation to known overriden methods

sky/sdk/lib/widgets/toggleable.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 '../animation/animated_value.dart';
 import '../animation/curves.dart';
 import 'animated_component.dart';
 import 'basic.dart';
@@ skipping 10 matching lines @@
     this.onChanged
   }) : super(key: key) {
     toggleAnimation = new AnimatedValue(value ? 1.0 : 0.0);
     watch(toggleAnimation);
   }
 
   bool value;
   AnimatedValue toggleAnimation;
   ValueChanged onChanged;
 
+  @override
   void syncFields(Toggleable source) {
     onChanged = source.onChanged;
     if (value != source.value) {
       value = source.value;
       double targetValue = value ? 1.0 : 0.0;
       double difference = (toggleAnimation.value - targetValue).abs();
       if (difference > 0) {
         toggleAnimation.stop();
         double t = difference * duration;
         Curve curve = targetValue > toggleAnimation.value ? curveUp : curveDown;
@@ skipping 10 matching lines @@
   // Override these methods to draw yourself
   void customPaintCallback(sky.Canvas canvas, Size size) {
     assert(false);
   }
   Size get size => Size.zero;
   EdgeDims get margin => const EdgeDims.symmetric(horizontal: 5.0);
   double get duration => 200.0;
   Curve get curveUp => easeIn;
   Curve get curveDown => easeOut;
 
+  @override
   Widget build() {
     return new Listener(
       child: new Container(
         margin: margin,
         width: size.width,
         height: size.height,
         child: new CustomPaint(
           token: toggleAnimation.value,
           callback: customPaintCallback
         )
       ),
       onGestureTap: _handleClick
     );
   }
 }