Make UINode's key a String

sky/sdk/lib/widgets/floating_action_button.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 '../theme2/colors.dart';
 import '../theme2/edges.dart';
 import 'basic.dart';
 import 'button_base.dart';
 import 'ink_well.dart';
 import 'material.dart';
 
 // TODO(eseidel): This needs to change based on device size?
 // http://www.google.com/design/spec/layout/metrics-keylines.html#metrics-keylines-keylines-spacing
 const double _kSize = 56.0;
 
 class FloatingActionButton extends ButtonBase {
 
-  FloatingActionButton({ Object key, this.child })
-      : super(key: key);
+  FloatingActionButton({ String key, this.child }) : super(key: key);
 
   final UINode child;
 
   UINode buildContent() {
     return new Material(
       child: new ClipOval(
         child: new Container(
           width: _kSize,
           height: _kSize,
           child: new InkWell(child: new Center(child: child))
         )
       ),
       color: Red[500],
       edge: MaterialEdge.circle,
       level: highlight ? 3 : 2
     );
   }
 
 }