Fixes issue 3474. If a method in an async RPC interface returns a Request or a RequestBuilder, previously ProxyCreator would generate uncompiling code. Now, such methods always return a Request or RequestBuilder, albeit sometimes a dummy stand-in object that always fails. Review by: jat, scottb, bruce git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.6@5023 8db76d5a-ed1c-0410-87a9-c151d255dfc7