Public: custom serializer and other code needed to send constraint violations
from the server to the client.

Review at http://gwt-code-reviews.appspot.com/1040802

Review by: bobv@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9135 8db76d5a-ed1c-0410-87a9-c151d255dfc7