Fixed custom elements to work in Polymer

tools/dom/src/dartium_CustomElementSupport.dart

Index: tools/dom/src/dartium_CustomElementSupport.dart
diff --git a/tools/dom/src/dartium_CustomElementSupport.dart b/tools/dom/src/dartium_CustomElementSupport.dart
index a14e07f114f012e3a43b1ff96e4a9d306dec1e8d..d80f2c45f571d693e83fe5758a34cfcc54bd2a23 100644
--- a/tools/dom/src/dartium_CustomElementSupport.dart
+++ b/tools/dom/src/dartium_CustomElementSupport.dart
@@ -26,11 +26,12 @@ class _VMElementUpgrader implements ElementUpgrader {
     }
   }
 
-  Element upgrade(Element element) {
-    if (element.runtimeType != _nativeType) {
+  Element upgrade(element) {
+    if (element.runtimeType != js.JsObjectImpl) {
       throw new UnsupportedError('Element is incorrect type');
     }
-    return _Utils.changeElementWrapper(element, _type);
+
+    return createCustomUpgrader(_nativeType, element);
   }
 }