Fix errors in Sky detected by Dart analyzer

sky/framework/components/button.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/shadows.dart';
 import 'material.dart';
 
 class Button extends Component {
   static final Style _style = new Style('''
     display: inline-flex;
     transform: translateX(0);
     -webkit-user-select: none;
     justify-content: center;
     align-items: center;
     height: 36px;
     min-width: 64px;
     padding: 0 8px;
     margin: 4px;
     border-radius: 2px;'''
   );
 
   Node content;
   int level;
 
   Button({ Object key, this.content, this.level }) : super(key: key);
 
   Node build() {
     return new Material(
       styles: [_style],
       children: [content],
       level: level
     );
   }
 }