Per suggestion from rsc.

Change-Id: I4b61ec6f35ffaaa792b75e011fbba1bdfbabc1f6
Reviewed-on: https://go-review.googlesource.com/37501
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>