Make UINode's key a String

sky/examples/stocks2/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/fixed_height_scrollable.dart';
 import 'package:sky/widgets/basic.dart';
 
 import 'stock_data.dart';
 import 'stock_row.dart';
 
 class Stocklist extends FixedHeightScrollable {
 
   Stocklist({
-    Object key,
+    String key,
     this.stocks,
     this.query
   }) : super(itemHeight: StockRow.kHeight, key: key);
 
   String query;
   List<Stock> stocks;
 
   void syncFields(Stocklist source) {
     query = source.query;
     stocks = source.stocks;
     super.syncFields(source);
   }
 
   List<UINode> buildItems(int start, int count) {
     var filteredStocks = stocks.where((stock) {
       return query == null ||
              stock.symbol.contains(new RegExp(query, caseSensitive: false));
     });
     itemCount = filteredStocks.length;
     return filteredStocks
       .skip(start)
       .take(count)
       .map((stock) => new StockRow(stock: stock))
       .toList(growable: false);
   }
 }