Introduced by (and missed during code review of) golang.org/cl/19847.

Change-Id: I03b76f36e5da69c31730380592dfa1c32570e17f
Reviewed-on: https://go-review.googlesource.com/19912
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>