Run attached test.sh.  Note that compiler complains about missing Ann.class but not
about missing Impl.class.  There's no reason the compiler should need the annotation
class file to compile code using the library.

If you remove the "message" method from Ann.java, the compiler won't complain about
the missing Ann.class file.

The compiler should *not* warn in either case.  Annotations used in the implementation
of a library should not impact users of the library.  This is consistent with how unknown
annotations are handled at runtime.