Only validate a constraint annotation once.

[JSR 303 TCK Result] 106 of 257 (41.25%) Pass with 24 Failures and 9 Errors.

Review at http://gwt-code-reviews.appspot.com/1373802

Review by: rchandia@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9791 8db76d5a-ed1c-0410-87a9-c151d255dfc7