Comment by j...@google.com: This looks like http://code.google.com/p/google-web-toolkit/issues/detail?id=4254 -- for now, you will have to use a later JDK.
For more information: http://code.google.com/p/google-web-toolkit/wiki/TroubleshootingOOPHM -- http://groups.google.com/group/Google-Web-Toolkit-Contributors