Add RenderClip and use it in FixedHeightScrollable

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import '../animation/scroll_behavior.dart';
 import '../fn2.dart';
 import 'dart:async';
 import 'dart:math' as math;
 import 'package:vector_math/vector_math.dart';
 import 'scrollable.dart';
@@ skipping 52 matching lines @@
         if (alignmentDelta != 0.0)
           alignmentDelta -= _itemHeight;
 
         double drawStart = scrollOffset + alignmentDelta;
         itemNumber = math.max(0, (drawStart / _itemHeight).floor());
 
         transform.translate(0.0, alignmentDelta);
       }
     }
 
-    // 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))
+      )
     );
   }
 
   List<UINode> buildItems(int start, int count);  
 }