Omit extra argument to createElement/NS if null

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

Index: tools/dom/templates/html/impl/impl_Document.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Document.darttemplate b/tools/dom/templates/html/impl/impl_Document.darttemplate
index d803f8e783d43ee32eacb9a84da34b4f6e9f0b1d..fb6d6b1889059268f5cada3b37301a8fb03ecddd 100644
--- a/tools/dom/templates/html/impl/impl_Document.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Document.darttemplate
@@ -64,7 +64,11 @@ $endif
   @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) :
@@ -86,18 +90,35 @@ $else
 $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;
@@ -107,7 +128,7 @@ $else
         wrapped = wrap_jso_custom_element(newElement);
       }
     }
-  
+
     return wrapped;
 $endif
   }