non fatal js exception escapes in IE9 devmode.
http://code.google.com/p/google-web-toolkit/issues/detail?id=6135

Review at http://gwt-code-reviews.appspot.com/1387802

Review by: jlabanca@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9861 8db76d5a-ed1c-0410-87a9-c151d255dfc7