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 fc9770624621cce7a70e2df720b7384c24c95299..7bce37aa7ecce79d941dd082cb874149a6e27a2f 100644
--- a/sky/sdk/lib/framework/fn2.dart
+++ b/sky/sdk/lib/framework/fn2.dart
@@ -441,6 +441,20 @@ class SizeObserver extends OneChildRenderObjectWrapper {
   }
 }
 
+class CustomPaint extends OneChildRenderObjectWrapper {

[abarth-chromium, 2015/06/04 23:58:39]

Can you add a TODO(jackson) to this class about needing a mechanism for marking
the RenderCustomPaint object as needing paint?

[jackson, 2015/06/05 00:23:10]

Done.

+  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;
+  }
+}

[abarth-chromium, 2015/06/04 23:58:39]

You probably want to implement a _remove similar to  SizeObserver to null out
the callback when this object is removed from the component hierarchy.

[jackson, 2015/06/05 00:23:10]

Done.

 
 final List<UINode> _emptyList = new List<UINode>();