Some tests have invalid cast errors

[stereotype441]

I will soon submit a CL that causes the front end to detect "invalid cast" errors in circumstances where an implicit downcast would usually be inserted, but the downcast can be statically proven to fail. (This is a feature of strong mode that is carried over from DDC).

This causes a few tests in language_2, lib_2, and standalone_2 to start failing with CompileTimeError. I'll tag the failing tests with this issue number.