Move example fn widgets into sky/framework/components

sky/framework/components/menu_item.dart

-part of widgets;
+// 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 'button_base.dart';
+import 'icon.dart';
 
 class MenuItem extends ButtonBase {
-
-  static Style _style = new Style('''
+  static final Style _style = new Style('''
     transform: translateX(0);
     display: flex;
     align-items: center;
     height: 48px;
     -webkit-user-select: none;'''
   );
 
-  static Style _highlightStyle = new Style('''
+  static final Style _highlightStyle = new Style('''
     transform: translateX(0);
     display: flex;
     align-items: center;
     height: 48px;
     background: rgba(153, 153, 153, 0.4);
     -webkit-user-select: none;'''
   );
 
-  static Style _iconStyle = new Style('''
+  static final Style _iconStyle = new Style('''
     padding: 0px 16px;'''
   );
 
-  static Style _labelStyle = new Style('''
+  static final Style _labelStyle = new Style('''
       font-family: 'Roboto Medium', 'Helvetica';
       color: #212121;
       padding: 0px 16px;
       flex: 1;'''
   );
 
   List<Node> children;
   String icon;
 
   MenuItem({ Object key, this.icon, this.children }) : super(key: key);
 
   Node build() {
     return new Container(
-      style: _highlight ? _highlightStyle : _style,
+      style: highlight ? _highlightStyle : _style,
       children: [
         super.build(),
         new Icon(
           style: _iconStyle,
           size: 24,
           type: "${icon}_grey600"
         ),
         new Container(
           style: _labelStyle,
           children: children
         )
       ]
     );
   }
 }