Fixed by CL 31092 already, but that change is a few steps away
from the problem observed here, so add an explicit test.

Fixes #17019.

Change-Id: If4ece1418e6596b1976961347889ce12c5969637
Reviewed-on: https://go-review.googlesource.com/31466
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>