Split layout2.dart into several files

sky/examples/stocks2/lib/stock_arrow.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 'dart:math';
 import 'package:sky/framework/fn2.dart';
-import 'package:sky/framework/layout2.dart';
 
 class StockArrow extends Component {
   static final Style _style = new Style('''
     width: 40px;
     height: 40px;
     align-items: center;
     justify-content: center;
     border-radius: 40px;
     margin-right: 16px;
     border: 1px solid transparent;'''
@@ skipping 66 matching lines @@
       style: _style,
       children: [
         new Container(
           inlineStyle: 'border-$type-color: $border',
           style: up ? _upStyle : _downStyle
         )
       ]
     );
   }
 }