Fixes issue #2054; AbstractCollection.removeAll() produced incorrect results when the collection contained duplicates.

Patch by: me (fix), sgross (test case)
Review by: spoon (desk)


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@2537 8db76d5a-ed1c-0410-87a9-c151d255dfc7