Omit extra argument to createElement/NS if null

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of $LIBRARYNAME;
 
 @DocsEditable()
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME extends Node
 {
 
@@ skipping 46 matching lines @@
 $endif
   }
 
   /// *Deprecated*: use [supportsRegisterElement] instead.
   @deprecated
   bool get supportsRegister => supportsRegisterElement;
 
   @DomName('Document.createElement')
   Element createElement(String tagName, [String typeExtension]) {
 $if DART2JS
-    return _createElement(tagName, typeExtension);
+    if (typeExtension == null) {
+      return _createElement_2(tagName);
+    } else {
+      return _createElement(tagName, typeExtension);
+    }
 $else
     var newElement = (typeExtension == null) ?
       _blink.BlinkDocument.instance.createElement_Callback_1_(unwrap_jso(this), tagName) :
       _blink.BlinkDocument.instance.createElement_Callback_2_(unwrap_jso(this), tagName, typeExtension);
 
     var wrapped;
 
     if (newElement['dart_class'] != null) {
       wrapped = newElement['dart_class'];         // Here's our Dart class.
       wrapped.blink_jsObject = newElement;
     } else {
       wrapped = wrap_jso(newElement);
       if (wrapped == null) {
         wrapped = wrap_jso_custom_element(newElement);
       }
     }
 
     return wrapped;
 $endif
   }
 
+$if DART2JS
+  // The two-argument version of this is automatically generated, but we need to
+  // omit the typeExtension if it's null on Firefox or we get an is="null" attribute.
+  @DomName('Document.createElement')
+  _createElement_2(String tagName) => JS('', '#.createElement(#)', this, tagName);
+
+  // The three-argument version of this is automatically generated, but we need to
+  // omit the typeExtension if it's null on Firefox or we get an is="null" attribute.
+  @DomName('Document.createElementNS')
+  _createElementNS_2(String namespaceURI, String qualifiedName) =>
+      JS('', '#.createElementNS(#, #)', this, namespaceURI, qualifiedName);
+
+$endif
   @DomName('Document.createElementNS')
   @DocsEditable()
   Element createElementNS(String namespaceURI, String qualifiedName, [String typeExtension]) {
 $if DART2JS
-    return _createElementNS(namespaceURI, qualifiedName, typeExtension);
+    if (typeExtension == null) {
+      return _createElementNS_2(namespaceURI, qualifiedName);
+    } else {
+      return _createElementNS(namespaceURI, qualifiedName, typeExtension);
+    }
 $else
     var newElement = (typeExtension == null) ?
       _blink.BlinkDocument.instance.createElementNS_Callback_2_(unwrap_jso(this), namespaceURI, qualifiedName) :
       _blink.BlinkDocument.instance.createElementNS_Callback_3_(unwrap_jso(this), namespaceURI, qualifiedName, typeExtension);
-  
+
     var wrapped;
-  
+
     if (newElement['dart_class'] != null) {
       wrapped = newElement['dart_class'];         // Here's our Dart class.
       wrapped.blink_jsObject = newElement;
     } else {
       wrapped = wrap_jso(newElement);
       if (wrapped == null) {
         wrapped = wrap_jso_custom_element(newElement);
       }
     }
-  
+
     return wrapped;
 $endif
   }
 
 $if DART2JS
   @DomName('Document.createNodeIterator')
   NodeIterator _createNodeIterator(Node root,
       [int whatToShow, NodeFilter filter])
       => JS('NodeIterator', '#.createNodeIterator(#, #, #, false)',
           this, root, whatToShow, filter);
 
   @DomName('Document.createTreeWalker')
   TreeWalker _createTreeWalker(Node root,
       [int whatToShow, NodeFilter filter])
       => JS('TreeWalker', '#.createTreeWalker(#, #, #, false)',
           this, root, whatToShow, filter);
 $endif
 }