Per code review feedback on CL 40693.

Change-Id: I38c522022a3c2f3e61ea90181391edb5c178916e
Reviewed-on: https://go-review.googlesource.com/41300
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@golang.org>