Type extension support in polyfill.

tools/dom/templates/html/impl/impl_Element.darttemplate

Index: tools/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Element.darttemplate b/tools/dom/templates/html/impl/impl_Element.darttemplate
index 71a481c199f0dc854373fade16909783b01bfeac..29e5621d4c915f183ed3194ec1877a965569a905 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -343,8 +343,8 @@ $(ANNOTATIONS)abstract class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
    *
    * * [isTagSupported]
    */
-  factory $CLASSNAME.tag(String tag) =>
-      _$(CLASSNAME)FactoryProvider.createElement_tag(tag);
+  factory $CLASSNAME.tag(String tag, [String typeExtention]) =>
+      _$(CLASSNAME)FactoryProvider.createElement_tag(tag, typeExtention);
 
   /// Creates a new `<a>` element.
   ///
@@ -661,7 +661,7 @@ $(ANNOTATIONS)abstract class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
    * 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);
   }
 
@@ -1305,12 +1305,12 @@ class _ElementFactoryProvider {
 $if DART2JS
   // Optimization to improve performance until the dart2js compiler inlines this
   // method.
-  static dynamic createElement_tag(String tag) =>
-      // Firefox may return a JS function for some types (Embed, Object).
-      JS('Element|=Object', 'document.createElement(#)', tag);
+  static dynamic createElement_tag(String tag, String typeExtension) =>
+    // Firefox may return a JS function for some types (Embed, Object).
+    JS('Element|=Object', 'document.createElement(#, #)', tag, typeExtension);
 $else
-  static Element createElement_tag(String tag) =>
-      document.$dom_createElement(tag);
+  static Element createElement_tag(String tag, String typeExtension) =>
+      document.$dom_createElement(tag, typeExtension);
 $endif
 }