Follow-up to https://golang.org/cl/22543.

Change-Id: I873b4fa6616ac2aea8faada2fccd126233bbc07f
Reviewed-on: https://go-review.googlesource.com/22583
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>