Makes vet happy.

Change-Id: I00de44a844afa57901b31991acf9e7ac834a9572
Reviewed-on: https://go-review.googlesource.com/32425
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>