Add an example of an app that manipulates both a RenderObject tree and has some fn logic in it.

sky/sdk/lib/framework/rendering/box.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' as math;
 import 'dart:sky' as sky;
 import 'object.dart';
 import '../painting/box_painter.dart';
 import 'package:vector_math/vector_math.dart';
 import 'package:sky/framework/net/image_cache.dart' as image_cache;
@@ skipping 230 matching lines @@
   Size _size = Size.zero;
   Size get size => _size;
   void set size(Size value) {
     assert(RenderObject.debugDoingLayout);
     _size = value;
   }
 
   String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}size: ${size}\n';
 }
 
-abstract class RenderProxyBox extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
-  RenderProxyBox(RenderBox child) {
+class RenderProxyBox extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+
+  RenderProxyBox([RenderBox child = null]) {
     this.child = child;
   }
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
     if (child != null)
       return child.getMinIntrinsicWidth(constraints);
     return super.getMinIntrinsicWidth(constraints);
   }
 
   double getMaxIntrinsicWidth(BoxConstraints constraints) {
@@ skipping 27 matching lines @@
     if (child != null)
       child.hitTest(result, position: position);
     else
       super.hitTestChildren(result, position: position);
   }
 
   void paint(RenderObjectDisplayList canvas) {
     if (child != null)
       child.paint(canvas);
   }
+
 }
 
 class RenderSizedBox extends RenderProxyBox {
 
   RenderSizedBox({
     RenderBox child,
     Size desiredSize: Size.infinite
   }) : super(child), _desiredSize = desiredSize {
     assert(desiredSize != null);
   }
@@ skipping 639 matching lines @@
 
   void defaultPaint(RenderObjectDisplayList canvas) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position);
       child = child.parentData.nextSibling;
     }
   }
 }