Add RenderClip and use it in FixedHeightScrollable

sky/sdk/lib/framework/components2/fixed_height_scrollable.dart

Index: sky/sdk/lib/framework/components2/fixed_height_scrollable.dart
diff --git a/sky/sdk/lib/framework/components2/fixed_height_scrollable.dart b/sky/sdk/lib/framework/components2/fixed_height_scrollable.dart
index 3a47f86bd044c6c82b6a7e401dd8187aac85f104..2cf747d2c3ed51a02a05913f6defec556b8a6e65 100644
--- a/sky/sdk/lib/framework/components2/fixed_height_scrollable.dart
+++ b/sky/sdk/lib/framework/components2/fixed_height_scrollable.dart
@@ -70,14 +70,11 @@ abstract class FixedHeightScrollable extends Scrollable {
       }
     }
 
-    // TODO(abarth): Add a clip.
-    return new BlockContainer(
-      children: [
-        new Transform(
-          transform: transform,
-          child: new BlockContainer(children: buildItems(itemNumber, drawCount))
-        )
-      ]
+    return new Clip(
+      child: new Transform(
+        transform: transform,
+        child: new BlockContainer(children: buildItems(itemNumber, drawCount))
+      )
     );
   }