[polymer] interop with polymer-element and polymer.js

pkg/web_components/lib/dart_support.js

 // Copyright (c) 2014, 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.
 
 (function() {
   var ShadowDOMPolyfill = window.ShadowDOMPolyfill;
   if (!ShadowDOMPolyfill) return;
 
+  if (navigator.userAgent.indexOf('(Dart)') !== -1) {
+    console.error("ShadowDOMPolyfill polyfill was loaded in Dartium. This " +
+        "will not work. This indicates that Dartium's Chrome version is " +
+        "not compatible with this version of web_components.");
+  }
+
   var needsConstructorFix = window.constructor === window.Window;
 
   // TODO(jmesserly): we need to wrap document somehow (a dart:html hook?)
   window.dartExperimentalFixupGetTag = function(originalGetTag) {
     var NodeList = ShadowDOMPolyfill.wrappers.NodeList;
     var ShadowRoot = ShadowDOMPolyfill.wrappers.ShadowRoot;
     var unwrapIfNeeded = ShadowDOMPolyfill.unwrapIfNeeded;
     function getTag(obj) {
       // TODO(jmesserly): do we still need these?
       if (obj instanceof NodeList) return 'NodeList';
       if (obj instanceof ShadowRoot) return 'ShadowRoot';
       if (MutationRecord && (obj instanceof MutationRecord))
           return 'MutationRecord';
       if (MutationObserver && (obj instanceof MutationObserver))
           return 'MutationObserver';
 
       // TODO(jmesserly): this prevents incorrect interaction between ShadowDOM
       // and dart:html's <template> polyfill. Essentially, ShadowDOM is
       // polyfilling native template, but our Dart polyfill fails to detect this
       // because the unwrapped node is an HTMLUnknownElement, leading it to
       // think the node has no content.
       if (obj instanceof HTMLTemplateElement) return 'HTMLTemplateElement';
 
       var unwrapped = unwrapIfNeeded(obj);
-      if (needsConstructorFix || obj !== unwrapped) {
+      if (unwrapped && (needsConstructorFix || obj !== unwrapped)) {
         // Fix up class names for Firefox, or if using the minified polyfill.
         // dart2js prefers .constructor.name, but there are all kinds of cases
         // where this will give the wrong answer.
         var ctor = obj.constructor
         if (ctor === unwrapped.constructor) {
           var name = ctor._ShadowDOMPolyfill$cacheTag_;
           if (!name) {
             name = Object.prototype.toString.call(unwrapped);
             name = name.substring(8, name.length - 1);
             ctor._ShadowDOMPolyfill$cacheTag_ = name;
           }
           return name;
         }
 
         obj = unwrapped;
       }
       return originalGetTag(obj);
     }
 
     return getTag;
   };
 })();