For #10571.

Change-Id: I9a42226078b9c52dbe0c65cb101b5f452233e911
Reviewed-on: https://go-review.googlesource.com/18205Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>