Add an implict animation between ActionBar backgrounds

sky/framework/components/action_bar.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 '../fn.dart';
 import '../theme/shadows.dart';
 import '../theme/view-configuration.dart';
 
 class ActionBar extends Component {
   static final Style _style = new Style('''
     display: flex;
     align-items: center;
     height: 56px;
     padding: 0 8px;
+    transition: background-color 0.3s;
     padding-top: ${kStatusBarHeight}px;
     box-shadow: ${Shadow[2]};''');
 
   static Style _centerStyle = new Style('''
     padding-left: 24px;
     flex: 1;''');
 
   Node left;
   Node center;
   List<Node> right;
 
   ActionBar({
     String key,
     this.left,
     this.center,
     this.right
   }) : super(key: key);
 
   Node build() {
     List<Node> children = [left, new StyleNode(center, _centerStyle)];
 
     if (right != null)
       children.addAll(right);
 
     return new Container(style: _style, children: children);
   }
 }