Make the popup menu work again.

sky/sdk/lib/widgets/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 'dart:math' as math;
 
 import 'package:vector_math/vector_math.dart';
 
 import '../framework/animation/scroll_behavior.dart';
 import 'basic.dart';
@@ skipping 50 matching lines @@
         if (alignmentDelta != 0.0)
           alignmentDelta -= itemHeight;
 
         double drawStart = scrollOffset + alignmentDelta;
         itemShowIndex = math.max(0, (drawStart / itemHeight).floor());
 
         transform.translate(0.0, alignmentDelta);
       }
     }
 
+    List<Widget> items = buildItems(itemShowIndex, itemShowCount);
+    assert(items.every((item) => item.key != null));
+
     return new SizeObserver(
       callback: _handleSizeChanged,
       child: new ClipRect(
         child: new Transform(
           transform: transform,
-          child: new Block(buildItems(itemShowIndex, itemShowCount))
+          child: new Block(items)
         )
       )
     );
   }
 
   List<Widget> buildItems(int start, int count);
 
 }