Create an example app that demonstrates interactive coordination of an fn tree and a raw RenderObje…

sky/sdk/lib/widgets/basic.dart

Index: sky/sdk/lib/widgets/basic.dart
diff --git a/sky/sdk/lib/widgets/basic.dart b/sky/sdk/lib/widgets/basic.dart
index 77b08538052db8d8d302f3042b23d2cbe7ee84e0..7137bb9d2ce0fe7876724174d3212c9d12f8ebac 100644
--- a/sky/sdk/lib/widgets/basic.dart
+++ b/sky/sdk/lib/widgets/basic.dart
@@ -307,7 +307,7 @@ class Positioned extends ParentDataNode {
     double right,
     double bottom,
     double left
-  }) : super(content,
+  }) : super(child,
              new StackParentData()..top = top
                                   ..right = right
                                   ..bottom = bottom
@@ -402,3 +402,30 @@ class Image extends RenderObjectWrapper {
   }
 
 }
+
+class UINodeToRenderBoxAdapter extends RenderObjectWrapper {
+
+  UINodeToRenderBoxAdapter(RenderBox renderBox)
+    : this.renderBox = renderBox,
+      super(key: renderBox.hashCode.toString());
+
+  RenderBox get root => super.root;
+  RenderBox createNode() => this.renderBox;
+
+  final RenderBox renderBox;
+
+  void syncRenderObject(UINode old) {
+    super.syncRenderObject(old);
+    if (old != null) {
+      assert(old is UINodeToRenderBoxAdapter);
+      assert(root == old.renderBox);
+    }
+  }
+
+  void insert(RenderObjectWrapper child, dynamic slot) {
+    assert(false);
+    // UINodeToRenderBoxAdapter cannot have UINode children; by
+    // definition, it is the transition out of the UINode world.
+  }
+
+}