Allows subclasses to use a different scheduler than Scheduler.get().

http://gwt-code-reviews.appspot.com/1385807/

Author: stephen.haberman
Review by: jlabanca


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9892 8db76d5a-ed1c-0410-87a9-c151d255dfc7