Pointed out during review of golang.org/cl/22104.

Change-Id: If8842e7f8146441e918ec6a2b6e893b7cf88615c
Reviewed-on: https://go-review.googlesource.com/22120
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>