Add @Override tags in order to be able to enable Eclipse warnings without being drowned

Review at http://gwt-code-reviews.appspot.com/754801

Review by: amitmanjhi@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8520 8db76d5a-ed1c-0410-87a9-c151d255dfc7