Switch to lookup declaration by tag name. This fixes issue 14274.

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 116 matching lines @@
   Job job(Job job, void callback(), Duration wait) =>
       runJob(job, callback, wait);
 
   void polymerCreated() {
     if (this.ownerDocument.window != null || alwaysPrepare ||
         _preparingElements > 0) {
       prepareElement();
     }
   }
 
+  /** Retrieves the custom element name by inspecting the host node. */
+  String get _customTagName {
+    var isAttr = attributes['is'];
+    return (isAttr == null || isAttr == '') ? localName : isAttr;
+  }
+
   void prepareElement() {
     // Dart note: get the _declaration, which also marks _elementPrepared
-    _declaration = _getDeclaration(this.runtimeType);
+    _declaration = _getDeclaration(_customTagName);
     // do this first so we can observe changes during initialization
     observeProperties();
     // install boilerplate attributes
     copyInstanceAttributes();
     // process input attributes
     takeAttributes();
     // add event listeners
     addHostListeners();
     // guarantees that while preparing, any sub-elements will also be prepared
     _preparingElements++;
@@ skipping 804 matching lines @@
 /**
  * Base class for PolymerElements deriving from HtmlElement.
  *
  * See [Polymer].
  */
 class PolymerElement extends HtmlElement with Polymer, Observable {
   PolymerElement.created() : super.created() {
     polymerCreated();
   }
 }