Fixing custom element type extensions in Dartium to have correct native type ID

tools/dom/src/native_DOMImplementation.dart

Index: tools/dom/src/native_DOMImplementation.dart
diff --git a/tools/dom/src/native_DOMImplementation.dart b/tools/dom/src/native_DOMImplementation.dart
index 037f4cd8807c3ce86b3273a2bf90a6892287b16b..cbea9eda1b4dc69f198b662d1de38e16057cb770 100644
--- a/tools/dom/src/native_DOMImplementation.dart
+++ b/tools/dom/src/native_DOMImplementation.dart
@@ -104,9 +104,12 @@ class _Utils {
     return false;
   }
 
-  static bool isTypeSubclassOfTag(Type type, String tagName) {
+  static Element getAndValidateNativeType(Type type, String tagName) {
     var element = new Element.tag(tagName);
-    return isTypeSubclassOf(type, element.runtimeType);
+    if (!isTypeSubclassOf(type, element.runtimeType)) {
+      return null;
+    }
+    return element;
   }
 
   static window() native "Utils_window";