Split layout2.dart into several files

sky/sdk/lib/framework/app.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 'layout2.dart';
 import 'dart:sky' as sky;
+import 'rendering/render_box.dart';
+import 'rendering/render_node.dart';
 
 class AppView {
 
   AppView(RenderBox root) {
     sky.view.setEventCallback(_handleEvent);
     sky.view.setBeginFrameCallback(_beginFrame);
 
     _renderView = new RenderView(child: root);
     _renderView.attach();
     _renderView.layout(new ViewConstraints(width: sky.view.width, height: sky.view.height));
@@ skipping 41 matching lines @@
         }
         break;
     }
     assert(result != null);
     result.path.reversed.forEach((RenderNode node) {
       node.handlePointer(event);
     });
   }
 
 }