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

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index a5382551aad5bed0b99e20209300349b7b8b1a3f..98482b7163159d7c918d1f6b66021b112e0f02cd 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -35350,9 +35350,12 @@ 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";