[Checkers] Immutability checker

[Michael Ernst]

Brian-

As a followup to my mention of our new @GuardedBy checker, we have for
several years had a checker for immutability -- essentially the same
semantics as JCIP's.  (You didn't seem to know about this when we talked at
JavaOne.)

That checker is distributed with the Checker Framework.  It's called the
IGJ checker, after a type system named IGJ that includes @Immutable and
other features (that you don't have to use if you don't want to).

You can find its documentation at

http://groups.csail.mit.edu/pag/jsr308/current/checkers-manual.html#igj-checker

                    -Mike