Avoid bottlenecks by using ConcurrentHashMap instead of a synchronized IdentityHashMap.

Repost from Rietveld issue 1582804
Thanks to Jason Terk for the original patch!
Review at http://gwt-code-reviews.appspot.com/1609803

Review by: unnurg@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10784 8db76d5a-ed1c-0410-87a9-c151d255dfc7