GWT validation refactoring; no user-visible changes.

Do a dependency-injection refactoring so that calls to get the threadlocal are
only in two places (in the two generators).

Review at http://gwt-code-reviews.appspot.com/1806804

Review by: nchalko@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11210 8db76d5a-ed1c-0410-87a9-c151d255dfc7