Make the popup menu width animation work

sky/sdk/example/stocks/lib/stock_list.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 'package:sky/widgets/scrollable_list.dart';
+import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/basic.dart';
 
 import 'stock_data.dart';
 import 'stock_row.dart';
 
 class Stocklist extends Component {
   Stocklist({ String key, this.stocks }) : super(key: key);
 
   final List<Stock> stocks;
 
   Widget build() {
-    return new ScrollableList<Stock>(
-      items: stocks,
-      itemHeight: StockRow.kHeight,
-      itemBuilder: (stock) => new StockRow(stock: stock)
+    return new Material(
+      type: MaterialType.canvas,
+      child: new ScrollableList<Stock>(
+        items: stocks,
+        itemHeight: StockRow.kHeight,
+        itemBuilder: (stock) => new StockRow(stock: stock)
+      )
     );
   }
 }