[Effen] remove some redundant styles

sky/framework/components/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 '../fn.dart';
+import '../layout.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);
-    display: flex;
-    flex-direction: row;
     justify-content: center;
     align-items: center;
     -webkit-user-select: none;
     cursor: pointer;
     width: 30px;
     height: 30px;'''
   );
 
   static final Style _containerStyle = new Style('''
     border: solid 2px;
@@ skipping 35 matching lines @@
   ValueChanged onChanged;
 
   Checkbox({ Object key, this.onChanged, this.checked }) : super(key: key);
 
   void _handleClick(sky.Event e) {
     onChanged(!checked);
   }
 
   UINode buildContent() {
     return new EventListenerNode(
-      new Container(
+      new FlexContainer(
         style: _style,
+        direction: FlexDirection.Row,
         children: [
           new Container(
             style: highlight ? _containerHighlightStyle : _containerStyle,
             children: [
               new Container(
                 style: checked ? _checkedStyle : _uncheckedStyle
               )
             ]
           )
         ]
       ),
       onGestureTap: _handleClick
     );
   }
 }