I had a similar experience with Jikes. There was a certain error in
the source code, but the compiler had a bug that caused it to terminate
abnormally with an assertion failure when it compiled that source,
rather than just issuing a message about the error and continuing. By
editing down the program that I was compiling to smaller and smaller
pieces, I eventually found the statement it was choking on, which was
indeed not valid Java.
  It may be that there is some error in the source you're compiling, an
error that your compiler isn't handling properly. Perhaps it is a
problem with a modifier in a field declaration. Try dividing your source
in half and seeing if you get the InternalError when compiling one of
the halves.

--Mike Amling