SI-7251, compiler crash with $.

We don't need to assert our way out of tight spots, we can issue
an error. Or so I once thought.

It turns out lots of assertions have been disappearing before
being heard thanks to "case t: Throwable". Under such conditions,
a failed assertion is a no-op, but an error is an error.

The crash associated with SI-7251 is best avoided by removing the
assertion, which allows an error to be issued in the normal course
of events.

In the course of trying to figure out the above, I cleaned up
ClassfileParser somewhat.
Merge pull request #2266 from paulp/issue/7251

SI-7251, compiler crash with $.
SKIPPED merge 2.10.x's #2266 into master.