This replaces r2963.  It adds a very narrow static eval rule for
JavaScript nodes: literal == null can be replaced by true or false
depending on what the literal is.

Review by: scottb (desk check)

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@2976 8db76d5a-ed1c-0410-87a9-c151d255dfc7