fix polymer copy-instance-attributes feature

pkg/polymer/lib/src/declaration.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of polymer;
 
 /**
  * **Deprecated**: use [Polymer.register] instead.
  *
  * Registers a [PolymerElement]. This is similar to [registerCustomElement]
@@ skipping 53 matching lines @@
 
   /** The names of published properties for this polymer-element. */
   Iterable<String> get publishedProperties =>
       _publish != null ? _publish.keys : const [];
 
   /** Same as [_publish] but with lower case names. */
   Map<String, DeclarationMirror> _publishLC;
 
   Map<String, Symbol> _observe;
 
-  Map<Symbol, Object> _instanceAttributes;
+  Map<String, Object> _instanceAttributes;
 
   List<Element> _sheets;
   List<Element> get sheets => _sheets;
 
   List<Element> _styles;
   List<Element> get styles => _styles;
 
   DocumentFragment get templateContent {
     final template = query('template');
     return template != null ? template.content : null;
@@ skipping 201 matching lines @@
       }
     }
 
     // NOTE: the following is not possible in Dart; fields must be declared.
     // install 'attributes' as properties on the prototype,
     // but don't override
   }
 
   void accumulateInstanceAttributes() {
     // inherit instance attributes
-    _instanceAttributes = new Map<Symbol, Object>();
+    _instanceAttributes = new Map<String, Object>();
     if (_super != null) _instanceAttributes.addAll(_super._instanceAttributes);
 
     // merge attributes from element
     attributes.forEach((name, value) {
       if (isInstanceAttribute(name)) {
-        _instanceAttributes[new Symbol(name)] = value;
+        _instanceAttributes[name] = value;
       }
     });
   }
 
   static bool isInstanceAttribute(name) {
     // do not clone these attributes onto instances
     final blackList = const {
         'name': 1, 'extends': 1, 'constructor': 1, 'noscript': 1,
         'attributes': 1};
 
@@ skipping 391 matching lines @@
   return map;
 }();
 
 // Dart note: we need this function because we have additional renames JS does
 // not have. The JS renames are simply case differences, whereas we have ones
 // like doubleclick -> dblclick and stripping the webkit prefix.
 String _eventNameFromType(String eventType) {
   final result = _reverseEventTranslations[eventType];
   return result != null ? result : eventType;
 }