[Checkers] Arrays and checkers

[Michael Ernst]

> Maybe we could instead do something like one of these:
> - 'the type "@Mutable String" is not permitted for the expression ...' 
> with found/required types as above
> - '@Mutable may not be applied to the type "@Immutable String"' where 
> "@Mutable" is the annotation(s) from the found type and "@Immutable 
> String" is the required type

I like the latter one better.  

(For the first one, I would be happier if you omitted "for the expression",
since the point is that "@Mutable String" is an illegal type, not that it's
illegal for a given expression.  But I still like the latter message
better.)

                    -Mike