Update Button to be made of Material

sky/framework/components/menu_divider.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';
 
 class MenuDivider extends Component {
   static final Style _style = new Style('''
     margin: 8px 0;
     border-bottom: 1px solid rgba(0, 0, 0, 0.12);'''
   );
 
   MenuDivider({ Object key }) : super(key: key);
 
   Node build() {
     return new Container(
-      style: _style
+      styles: [_style]
     );
   }
 }