Allow Effen Styles to be extendable

sky/framework/components/toolbar.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/colors.dart';
 import '../theme/shadows.dart';
 
 class Toolbar extends Component {
   List<Node> children;
 
   static final Style _style = new Style('''
     display: flex;
     align-items: center;
     height: 56px;
     z-index: 1;
     background-color: ${Purple[500]};
     color: white;
     box-shadow: ${Shadow[2]};'''
   );
 
   Toolbar({String key, this.children}) : super(key: key);
 
   Node build() {
     return new Container(
-      styles: [_style],
+      style: _style,
       children: children
     );
   }
 }