Better implementation for getHandlers (the old one threw an exception in Web mode)

Review at http://gwt-code-reviews.appspot.com/651801

Review by: fredsa@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8299 8db76d5a-ed1c-0410-87a9-c151d255dfc7