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

Index: sky/sdk/lib/framework/rendering/box.dart
diff --git a/sky/sdk/lib/framework/rendering/box.dart b/sky/sdk/lib/framework/rendering/box.dart
index f6bf0da0977043b5f84b285908bdd0ee37d90250..993916962b25a2cf4a376f04e2836b8877d7c223 100644
--- a/sky/sdk/lib/framework/rendering/box.dart
+++ b/sky/sdk/lib/framework/rendering/box.dart
@@ -248,8 +248,9 @@ abstract class RenderBox extends RenderObject {
   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;
   }
 
@@ -297,6 +298,7 @@ abstract class RenderProxyBox extends RenderBox with RenderObjectWithChildMixin<
     if (child != null)
       child.paint(canvas);
   }
+
 }
 
 class RenderSizedBox extends RenderProxyBox {