For #17496.

Change-Id: I671a59581c54d17bc272767eeb7b2742b54eca38
Reviewed-on: https://go-review.googlesource.com/32183
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>