Sync DOMImplementation IDL with spec

LayoutTests/dom/xhtml/level3/core/documentgetdoctype01.js

 
 /*
 Copyright © 2001-2004 World Wide Web Consortium, 
 (Massachusetts Institute of Technology, European Research Consortium 
 for Informatics and Mathematics, Keio University). All 
 Rights Reserved. This work is distributed under the W3C® Software License [1] in the 
 hope that it will be useful, but WITHOUT ANY WARRANTY; without even 
 the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 
 
 [1] http://www.w3.org/Consortium/Legal/2002/copyright-software-20021231
@@ skipping 113 matching lines @@
           throw ex;
           }
        } else { 
        throw ex;
         }
          }
         replacedDocType = doc.doctype;
 
       newSysID = replacedDocType.systemId;
 
-      assertNull("newSysIdNull",newSysID);
+      assertEquals("newSysIdNull", "null", newSysID);
     
 }
 
 
 
 
 function runTest() {
    documentgetdoctype01();
 }