make it possible to just use devmode on a particular module while allowing the
others to run in prod mode

Review at http://gwt-code-reviews.appspot.com/1408802

Review by: fabiomfv@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9972 8db76d5a-ed1c-0410-87a9-c151d255dfc7