fix polymer copy-instance-attributes feature

pkg/polymer/lib/src/instance.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;
 
 /**
  * Use this annotation to publish a field as an attribute. For example:
  *
  *     class MyPlaybackElement extends PolymerElement {
@@ skipping 255 matching lines @@
   // TODO(jmesserly): use stream or future here?
   void onMutation(Node node, void listener(MutationObserver obs)) {
     new MutationObserver((records, MutationObserver observer) {
       listener(observer);
       observer.disconnect();
     })..observe(node, childList: true, subtree: true);
   }
 
   void copyInstanceAttributes() {
     _declaration._instanceAttributes.forEach((name, value) {
-      attributes[name] = value;
+      attributes.putIfAbsent(name, () => value);
     });
   }
 
   void takeAttributes() {
     if (_declaration._publishLC == null) return;
     attributes.forEach(attributeToProperty);
   }
 
   /**
    * If attribute [name] is mapped to a property, deserialize
@@ skipping 634 matching lines @@
 }
 
 final Logger _observeLog = new Logger('polymer.observe');
 final Logger _eventsLog = new Logger('polymer.events');
 final Logger _unbindLog = new Logger('polymer.unbind');
 final Logger _bindLog = new Logger('polymer.bind');
 
 final Expando _shadowHost = new Expando<Element>();
 
 final Expando _eventHandledTable = new Expando<Set<Node>>();