Add RenderClip and use it in FixedHeightScrollable

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 7d7ff51150f10062609eb5b27891046dcaddc0ee..cbbfc658aa42ecba1f7ec4a2af0eaab54d365dac 100644
--- a/sky/sdk/lib/framework/rendering/box.dart
+++ b/sky/sdk/lib/framework/rendering/box.dart
@@ -217,6 +217,19 @@ class RenderSizedBox extends RenderProxyBox {
   }
 }
 
+class RenderClip extends RenderProxyBox {
+  RenderClip({ RenderBox child }) : super(child);
+
+  void paint(RenderObjectDisplayList canvas) {
+    if (child != null) {
+      canvas.save();
+      canvas.clipRect(new sky.Rect.fromSize(size));
+      child.paint(canvas);
+      canvas.restore();
+    }
+  }
+}
+
 class RenderPadding extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
 
   RenderPadding({ EdgeDims padding, RenderBox child }) {