Ensure all ProxyAutoBeans not directly referenced from root object or via a shim can be garbage-collected.
Issue 6193.
Patch by: bobv
Suggested by: t.broyer
Review by: rjrjr

Review at http://gwt-code-reviews.appspot.com/1451819


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10344 8db76d5a-ed1c-0410-87a9-c151d255dfc7