Switch to internal implementation of StringInterner to avoid class loader
leaks when reloading the page in dev mode. (Not tuned.)

Review at http://gwt-code-reviews.appspot.com/1578804

Review by: tobyr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10725 8db76d5a-ed1c-0410-87a9-c151d255dfc7