Omit extra argument to createElement/NS if null

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 15fe25452613f8a368e953d3b0db3eac6f796d27..ece22d30944a33fa4abd59eb2bd24f11f3823bfd 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -9834,13 +9834,32 @@ class Document extends Node
 
   @DomName('Document.createElement')
   Element createElement(String tagName, [String typeExtension]) {
-    return _createElement(tagName, typeExtension);
+    if (typeExtension == null) {
+      return _createElement_2(tagName);
+    } else {
+      return _createElement(tagName, typeExtension);
+    }
   }
 
+  // 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);
+
   @DomName('Document.createElementNS')
   @DocsEditable()
   Element createElementNS(String namespaceURI, String qualifiedName, [String typeExtension]) {
-    return _createElementNS(namespaceURI, qualifiedName, typeExtension);
+    if (typeExtension == null) {
+      return _createElementNS_2(namespaceURI, qualifiedName);
+    } else {
+      return _createElementNS(namespaceURI, qualifiedName, typeExtension);
+    }
   }
 
   @DomName('Document.createNodeIterator')