Added Alan's CL 1398603003 and augmented with upgrade for HtmlElement and HTMLElement

tools/dom/src/dartium_CustomElementSupport.dart

 // 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.
 
 part of dart.dom.html;
 
 /// Dartium ElementUpgrader implementation.
 class _VMElementUpgrader implements ElementUpgrader {
   final Type _type;
   final Type _nativeType;
 
   _VMElementUpgrader(Document document, Type type, String extendsTag) :
       _type = type,
       _nativeType = _validateCustomType(type).reflectedType {
 
     if (extendsTag == null) {
       if (_nativeType != HtmlElement) {
         throw new UnsupportedError('Class must provide extendsTag if base '
               'native class is not HtmlElement');
       }
     } else {
       if (document.createElement(extendsTag).runtimeType != _nativeType) {
         throw new UnsupportedError(
             'extendsTag does not match base native class');
       }
     }
   }
 
   Element upgrade(element) {
-    if (element.runtimeType != js.JsObjectImpl) {
-      throw new UnsupportedError('Element is incorrect type');
+    var jsObject;
+    var tag;
+    if (element.runtimeType == HtmlElement) {
+      jsObject = unwrap_jso(element);
+      tag = element.localName;
+    } else if (element.runtimeType == js.JsObjectImpl) {
+      // It's a Polymer core element (written in JS).
+      jsObject = element;
+      tag = element['localName'];
+    } else {
+      throw new UnsupportedError('Element is incorrect type. Got ${element.runtimeType}, expected HtmlElement/JsObjectImpl.');
     }
 
     // Remember Dart class to tagName for any upgrading done in wrap_jso.
-    var tag = element['localName'];
     _knownCustomeElements[tag] = _type;
 
-    return createCustomUpgrader(_nativeType, element);
+    return createCustomUpgrader(_nativeType, jsObject);
   }
 }
 
 /// Validates that the custom type is properly formed-
 ///
 /// * Is a user-defined class.
 /// * Has a created constructor with zero args.
 /// * Derives from an Element subclass.
 ///
 /// Then returns the native base class.
@@ skipping 40 matching lines @@
   return nativeClass;
 }
 
 
 bool _isBuiltinType(ClassMirror cls) {
   // TODO(vsm): Find a less hackish way to do this.
   LibraryMirror lib = cls.owner;
   String libName = lib.uri.toString();
   return libName.startsWith('dart:');
 }