For #15810.

Change-Id: Ib529808f664392feb9b36770f3d3d875fcb54528
Reviewed-on: https://go-review.googlesource.com/23488
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>