Add a test for issue 5134 to ensure that RequestFactory interfaces can be extended.
Patch by: bobv
Review by: rchandia
Reported by: pjulien

Review at http://gwt-code-reviews.appspot.com/1138801


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9272 8db76d5a-ed1c-0410-87a9-c151d255dfc7