Fix covariant return types for methods declared in domain objects.
Issue 5512.
Patch by: bobv
Review by: rchandia

Review at http://gwt-code-reviews.appspot.com/1136801


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9270 8db76d5a-ed1c-0410-87a9-c151d255dfc7