Split layout2.dart into several files

sky/sdk/lib/framework/components2/menu_item.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 '../fn2.dart';
-import '../layout2.dart';
 import 'button_base.dart';
 import 'icon.dart';
 import 'ink_well.dart';
 
 class MenuItem extends ButtonBase {
   static final Style _style = new Style('''
     align-items: center;
     height: 48px;
     -webkit-user-select: none;'''
   );
@@ skipping 42 matching lines @@
               _labelFlex
             )
           ]
         ),
         highlight ? _highlightStyle : _style
       ),
       onGestureTap: onGestureTap
     );
   }
 }