Move example fn widgets into sky/framework/components

sky/framework/components/button.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';
 
 class Button extends ButtonBase {
-
-  static Style _style = new Style('''
+  static final Style _style = new Style('''
     transform: translateX(0);
     display: inline-flex;
     border-radius: 4px;
     justify-content: center;
     align-items: center;
     border: 1px solid blue;
     -webkit-user-select: none;
     margin: 5px;'''
   );
 
-  static Style _highlightStyle = new Style('''
+  static final Style _highlightStyle = new Style('''
     transform: translateX(0);
     display: inline-flex;
     border-radius: 4px;
     justify-content: center;
     align-items: center;
     border: 1px solid blue;
     -webkit-user-select: none;
     margin: 5px;
     background-color: orange;'''
   );
 
   Node content;
 
   Button({ Object key, this.content }) : super(key: key);
 
   Node build() {
     return new Container(
-      key: 'Button',
-      style: _highlight ? _highlightStyle : _style,
+      style: highlight ? _highlightStyle : _style,
       children: [super.build(), content]
     );
   }
 }