Fix IE11 issue: this complements the CL I sent last week. It turns out that

tools/dom/src/dart2js_CustomElementSupport.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 dart.dom.html;
 
 _callConstructor(constructor, interceptor) {
   return (receiver) {
     setNativeSubclassDispatchRecord(receiver, interceptor);
 
@@ skipping 29 matching lines @@
 _makeCallbackMethod3(callback) {
   return JS('',
       '''((function(invokeCallback) {
              return function(arg1, arg2, arg3) {
                return invokeCallback(this, arg1, arg2, arg3);
              };
           })(#))''',
       convertDartClosureToJS(callback, 4));
 }
 
+/// Checks whether the given [element] correctly extends from the native class
+/// with the given [baseClassName]. This method will throw if the base class
+/// doesn't match, except when the element extends from `template` and it's base
+/// class is `HTMLUnknownElement`. This exclusion is needed to support extension
+/// of template elements (used heavily in Polymer 1.0) on IE11 when using the
+/// webcomponents-lite.js polyfill.
+void _checkExtendsNativeClassOrTemplate(
+    Element element, String extendsTag, String baseClassName) {
+  if (!JS('bool', '(# instanceof window[#])', element, baseClassName) &&
+      !((extendsTag == 'template' &&
+       JS('bool', '(# instanceof window["HTMLUnknownElement"])', element)))) {
+    throw new UnsupportedError('extendsTag does not match base native class');
+  }
+}
+
 void _registerCustomElement(context, document, String tag, Type type,
     String extendsTagName) {
   // Function follows the same pattern as the following JavaScript code for
   // registering a custom element.
   //
   //    var proto = Object.create(HTMLElement.prototype, {
   //        createdCallback: {
   //          value: function() {
   //            window.console.log('here');
   //          }
@@ skipping 23 matching lines @@
   if (baseClassName == null) {
     throw new ArgumentError(type);
   }
 
   if (extendsTagName == null) {
     if (baseClassName != 'HTMLElement') {
       throw new UnsupportedError('Class must provide extendsTag if base '
           'native class is not HtmlElement');
     }
   } else {
-    if (!JS('bool', '(#.createElement(#) instanceof window[#])',
-        document, extendsTagName, baseClassName)) {
-      throw new UnsupportedError('extendsTag does not match base native class');
-    }
+    var element = document.createElement(extendsTagName);
+    _checkExtendsNativeClassOrTemplate(element, extendsTagName, baseClassName);
   }
 
   var baseConstructor = JS('=Object', '#[#]', context, baseClassName);
 
   var properties = JS('=Object', '{}');
 
   JS('void', '#.createdCallback = #', properties,
       JS('=Object', '{value: #}',
           _makeCallbackMethod(_callConstructor(constructor, interceptor))));
   JS('void', '#.attachedCallback = #', properties,
@@ skipping 49 matching lines @@
     }
 
     if (extendsTag == null) {
       if (baseClassName != 'HTMLElement') {
         throw new UnsupportedError('Class must provide extendsTag if base '
             'native class is not HtmlElement');
       }
       _nativeType = HtmlElement;
     } else {
       var element = document.createElement(extendsTag);
-      if (!JS('bool', '(# instanceof window[#])', element, baseClassName) &&
-        // Exception to support template elements (extended for core pieces of
-        // Polymer 1.0) when using the webcomponents-lite.js polyfill on IE11:
-        !((extendsTag == 'template' &&
-         JS('bool', '(# instanceof window["HTMLUnknownElement"])', element)))) {
-        throw new UnsupportedError(
-            'extendsTag does not match base native class');
-      }
+      _checkExtendsNativeClassOrTemplate(element, extendsTag, baseClassName);
       _nativeType = element.runtimeType;
     }
 
     _interceptor = JS('=Object', '#.prototype', interceptorClass);
   }
 
   Element upgrade(Element element) {
     // Only exact type matches are supported- cannot be a subclass.
     if (element.runtimeType != _nativeType) {
       throw new ArgumentError('element is not subclass of $_nativeType');
     }
 
     setNativeSubclassDispatchRecord(element, _interceptor);
     JS('', '#(#)', _constructor, element);
     return element;
   }
 }