Exposed a method to iterate over the immediate subclasses of a class.  When looking at subtypes don't reexaming fields that have already been look at during the subtype scan.  Doing so can lessen the precision when raw supertypes are involved.

Patch by: mmendez, spoon (pair prog)

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@2892 8db76d5a-ed1c-0410-87a9-c151d255dfc7