Update Button to be made of Material

sky/framework/components/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 '../fn.dart';
 
 const String kAssetBase = '/sky/assets/material-design-icons';
 
 class Icon extends Component {
-  Style style;
+  List<Style> styles;
   int size;
   String type;
 
   Icon({
     String key,
-    this.style,
+    this.styles,
     this.size,
     this.type: ''
   }) : super(key: key);
 
   Node build() {
     String category = '';
     String subtype = '';
     List<String> parts = type.split('/');
     if (parts.length == 2) {
       category = parts[0];
       subtype = parts[1];
     }
 
     return new Image(
-      style: style,
+      styles: styles,
       width: size,
       height: size,
       src: '${kAssetBase}/${category}/2x_web/ic_${subtype}_${size}dp.png'
     );
   }
 }