Split layout2.dart into several files

sky/examples/raw/touch_demo.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 'dart:sky';
 import 'package:sky/framework/app.dart';
-import 'package:sky/framework/layout2.dart';
+import 'package:sky/framework/rendering/render_box.dart';
+import 'package:sky/framework/rendering/render_node.dart';
 
 // Material design colors. :p
 List<int> colors = [
   0xFF009688,
   0xFFFFC107,
   0xFF9C27B0,
   0xFF03A9F4,
   0xFF673AB7,
   0xFFCDDC39,
 ];
@@ skipping 46 matching lines @@
       dot.paint(canvas);
     });
   }
 }
 
 AppView app;
 
 void main() {
   app = new AppView(new RenderTouchDemo());
 }