Refactor padding out of RenderBlock.

sky/examples/raw/simple_render_tree.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:sky';
 import 'package:sky/framework/app.dart';
 import 'package:sky/framework/layout2.dart';
 
 class RenderSolidColor extends RenderDecoratedBox {
   final double desiredHeight;
@@ skipping 38 matching lines @@
     child.parentData.flex = flex;
   }
 
   // Yellow bar at top
   addFlexChild(root, 0xFFFFFF00, flex: 1);
 
   // Turquoise box
   root.add(new RenderSolidColor(0x7700FFFF, desiredHeight: 100.0, desiredWidth: 100.0));
 
   // Green and cyan render block with padding
-  var renderBlock = new RenderBlock(
-      decoration: new BoxDecoration(backgroundColor: 0xFFFFFFFF),
-      padding: const EdgeDims(10.0, 10.0, 10.0, 10.0));
+  var renderBlock = new RenderBlock(decoration: new BoxDecoration(backgroundColor: 0xFFFFFFFF));
 
   renderBlock.add(new RenderSolidColor(0xFF00FF00, desiredHeight: 50.0, desiredWidth: 100.0));
   renderBlock.add(new RenderSolidColor(0x7700FFFF, desiredHeight: 100.0, desiredWidth: 50.0));
 
-  root.add(renderBlock);
+  root.add(new RenderPadding(const EdgeDims(10.0, 10.0, 10.0, 10.0), renderBlock));
 
   var row = new RenderFlex(
     direction: FlexDirection.Horizontal,
     decoration: new BoxDecoration(backgroundColor: 0xFF333333));
 
   // Purple and blue cells
   addFlexChild(row, 0x77FF00FF, flex: 1);
   addFlexChild(row, 0xFF0000FF, flex: 2);
 
   root.add(row);
   row.parentData.flex = 3;
 
   app = new AppView(root);
 
 }