Fix OpenJDK compatibility (a generic type inference bug) by removing the
ability to return a different type than is requested.  None of our code
uses this ability, and it is easy enough to copy the returned list into a
list of the supertype externally anyway.

Issue: 3058
Patch by: jat
Review by: scottb, bobv



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5184 8db76d5a-ed1c-0410-87a9-c151d255dfc7