Ensure that all base URL's are absolut-ify-ed, no matter where we get them from.
Also refactor the compute script base code to be easier to read

Review at http://gwt-code-reviews.appspot.com/1066801

Review by: amitmanjhi@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9180 8db76d5a-ed1c-0410-87a9-c151d255dfc7