StringKey hashCode inconsistent across VMs.

This is causing PersistentUnitCache to never re-use generated types.  The deserialized ContentId hashCodes don't match the in-memory ContentId hashCodes because getClass().hashCode() isn't stable across VM instances.

http://gwt-code-reviews.appspot.com/1435801/

Review by: zundel@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10149 8db76d5a-ed1c-0410-87a9-c151d255dfc7