Fix errors in Sky detected by Dart analyzer

sky/framework/components/floating_action_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 'material.dart';
 import '../theme/colors.dart';
-import '../theme/shadows.dart';
 
 class FloatingActionButton extends Component {
   static final Style _style = new Style('''
     position: absolute;
     bottom: 16px;
     right: 16px;
     z-index: 5;
     transform: translateX(0);
     width: 56px;
     height: 56px;
@@ skipping 35 matching lines @@
       children: [
         new Material(
           key: "Clip",
           styles: [_clipStyle],
           children: children
         )
       ]
     );
   }
 }