Fix external issue 4711 - java.sql.Timestamp#compareTo(java.util.Date) silently crashes in Firefox and Chrome

Review at http://gwt-code-reviews.appspot.com/634802

Review by: jat@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8362 8db76d5a-ed1c-0410-87a9-c151d255dfc7