For #9039.

Change-Id: I2b1bcd76857ff332411ca21a0cc5def3097a8eaf
Reviewed-on: https://go-review.googlesource.com/30936
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>