Split layout2.dart into several files

sky/sdk/lib/framework/components2/checkbox.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 'dart:sky' as sky;
 
 typedef void ValueChanged(value);
 
 class Checkbox extends ButtonBase {
   static final Style _style = new Style('''
     transform: translateX(0);
     justify-content: center;
     align-items: center;
@@ skipping 61 matching lines @@
                 style: checked ? _checkedStyle : _uncheckedStyle
               )
             ]
           )
         ]
       ),
       onGestureTap: _handleClick
     );
   }
 }