Make UINode's key a String

sky/sdk/lib/widgets/material.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 '../painting/box_painter.dart';
 import '../theme2/colors.dart';
 import '../theme2/edges.dart';
 import '../theme2/shadows.dart';
 import 'basic.dart';
 
 class Material extends Component {
 
   Material({
-    Object key,
+    String key,
     this.child,
     this.edge: MaterialEdge.card,
     this.level: 0,
     this.color
   }) : super(key: key);
 
   final UINode child;
   final int level;
   final MaterialEdge edge;
   final Color color;
 
   // TODO(ianh): we should make this animate level changes and color changes
 
   UINode build() {
     return new Container(
       decoration: new BoxDecoration(
         boxShadow: shadows[level],
         borderRadius: edges[edge],
         backgroundColor: color == null ? Grey[50] : color,
         shape: edge == MaterialEdge.circle ? Shape.circle : Shape.rectangle
       ),
       child: child
     );
   }
 
 }