This fixes `go test go/types`.

https://golang.org/cl/23487/ introduced this code which contains
two unused variables (declared and assigned to, but never read).
cmd/compile doesn't report the error due open issue #8560 (the
variables are assigned to in a closure), but go/types does. The
build bot only runs go/types tests in -short mode (which doesn't
typecheck the std lib), hence this doesn't show up on the dashboard
either.

We cannot call b.Fatal and friends in the goroutine. Communicating
the error to the invoking function requires a channel or a mutex.
Unless the channel/sycnhronized variable is tested in each iteration
that follows, the iteration blocks if there's a failure. Testing in
each iteration may affect benchmark times.

One could use a time-out but that time depends on the underlying system.
Panicking seems good enough in this unlikely case; better than hanging
or affecting benchmark times.

Change-Id: Idce1172da8058e580fa3b3e398825b0eb4316325
Reviewed-on: https://go-review.googlesource.com/23528
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>