Add RenderClip and use it in FixedHeightScrollable

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 d5622c6a5779602bda9d97284e33b1287880f73b..1a345c8a27e962efde2b493b26b7ad4ac5591ca6 100644
--- a/sky/sdk/lib/framework/fn2.dart
+++ b/sky/sdk/lib/framework/fn2.dart
@@ -352,6 +352,15 @@ abstract class OneChildRenderObjectWrapper extends RenderObjectWrapper {
   }
 }
 
+class Clip extends OneChildRenderObjectWrapper {
+  RenderClip root;
+
+  Clip({ UINode child, Object key })
+    : super(child: child, key: key);
+
+  RenderClip createNode() => new RenderClip();
+}
+
 class Padding extends OneChildRenderObjectWrapper {
   RenderPadding root;
   final EdgeDims padding;