Passes toolstash -cmp.

Change-Id: Ie41d7e74847c44a8fd174731374339c6c32b1460
Reviewed-on: https://go-review.googlesource.com/21231
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>