Add @override annotation to known overriden methods

sky/sdk/lib/widgets/icon.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 '../mojo/asset_bundle.dart';
 import 'basic.dart';
 
 AssetBundle _initIconBundle() {
   if (rootBundle != null)
     return rootBundle;
   const String _kAssetBase = '/packages/sky/assets/material-design-icons/';
   return new NetworkAssetBundle(Uri.base.resolve(_kAssetBase));
 }
 
 final AssetBundle _iconBundle = _initIconBundle();
 
 class Icon extends Component {
   Icon({ String key, this.size, this.type: '' }) : super(key: key);
 
   final int size;
   final String type;
 
+  @override
   Widget build() {
     String category = '';
     String subtype = '';
     List<String> parts = type.split('/');
     if (parts.length == 2) {
       category = parts[0];
       subtype = parts[1];
     }
     // TODO(eseidel): This clearly isn't correct.  Not sure what would be.
     // Should we use the ios images on ios?
     String density = 'drawable-xxhdpi';
     return new AssetImage(
       bundle: _iconBundle,
       name: '${category}/${density}/ic_${subtype}_${size}dp.png',
       size: new Size(size.toDouble(), size.toDouble())
     );
   }
 }