Type extension support in polyfill.

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 1792f3a5b27ba3db33e04f7242303bcca0f64229..74d1a0147a7bf086a12a089db9f0a1041b9b749d 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -9365,8 +9365,8 @@ abstract class Element extends Node implements ParentNode, ChildNode {
    *
    * * [isTagSupported]
    */
-  factory Element.tag(String tag) =>
-      _ElementFactoryProvider.createElement_tag(tag);
+  factory Element.tag(String tag, [String typeExtention]) =>
+      _ElementFactoryProvider.createElement_tag(tag, typeExtention);
 
   /// Creates a new `<a>` element.
   ///
@@ -9683,7 +9683,7 @@ abstract class Element extends Node implements ParentNode, ChildNode {
    * The tag should be a valid HTML tag name.
    */
   static bool isTagSupported(String tag) {
-    var e = _ElementFactoryProvider.createElement_tag(tag);
+    var e = _ElementFactoryProvider.createElement_tag(tag, null);
     return e is Element && !(e is UnknownElement);
   }
 
@@ -10945,8 +10945,8 @@ class _ElementFactoryProvider {
   }
 
   @DomName('Document.createElement')
-  static Element createElement_tag(String tag) =>
-      document.$dom_createElement(tag);
+  static Element createElement_tag(String tag, String typeExtension) =>
+      document.$dom_createElement(tag, typeExtension);
 }