[Checkers] Interned checker throws ClassCastException

[Michael Ernst]

Mahmood-

Thanks for the fixes!  I appreciate it.  The test suites work for me, and
the Interned checker also runs on all of Daikon.  (It takes an hour on
my laptop, and 10 minutes on my desktop machine; at some point we should
figure out why it's so slow, but correctness is more important.

I've checked in a few tests for the Interned checker, so it is failing
a few of its unit tests again.

I'm glad to see the checkers achieving a much greater level of robustness!

                    -Mike