Not a fix but useful for further debugging, and safe.

For #18643.

Change-Id: I5fb4f4a8662007a26e945fff3986347855f00eab
Reviewed-on: https://go-review.googlesource.com/46393
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>