Implement radio button for Sky’s fn2

sky/sdk/lib/framework/fn2.dart

Index: sky/sdk/lib/framework/fn2.dart
diff --git a/sky/sdk/lib/framework/fn2.dart b/sky/sdk/lib/framework/fn2.dart
index fd39b82050935f4e5c8846e7709a912159eb73fb..540566963cde71d230207b2a5d2655d0a54d4a5a 100644
--- a/sky/sdk/lib/framework/fn2.dart
+++ b/sky/sdk/lib/framework/fn2.dart
@@ -441,6 +441,26 @@ class SizeObserver extends OneChildRenderObjectWrapper {
   }
 }
 
+// TODO(jackson) need a mechanism for marking the RenderCustomPaint as needing paint
+class CustomPaint extends OneChildRenderObjectWrapper {
+  RenderCustomPaint root;
+  final CustomPaintCallback callback;
+
+  CustomPaint({ this.callback, UINode child, Object key })
+    : super(child: child, key: key);
+
+  RenderCustomPaint createNode() => new RenderCustomPaint(callback: callback);
+
+  void syncRenderObject(CustomPaint old) {
+    super.syncRenderObject(old);
+    root.callback = callback;
+  }
+
+  void _remove() {
+    root.callback = null;
+    super._remove();
+  }
+}
 
 final List<UINode> _emptyList = new List<UINode>();