Make UINode's key a String

sky/sdk/lib/editing2/editable_text.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:async';
 
 import '../widgets/basic.dart';
 import 'editable_string.dart';
 
 class EditableText extends Component {
 
-  EditableText({Object key, this.value, this.focused})
+  EditableText({String key, this.value, this.focused})
       : super(key: key, stateful: true);
 
   // static final Style _cursorStyle = new Style('''
   //   width: 2px;
   //   height: 1.2em;
   //   vertical-align: top;
   //   background-color: ${Blue[500]};'''
   // );
 
   // static final Style _composingStyle = new Style('''
@@ skipping 68 matching lines @@
     //   children.add(new Container(
     //     key: 'cursor',
     //     // style: _cursorStyle
     //     ));
 
     return new Paragraph(
       text: hack
     );
   }
 }