Replaces CL 33356.

Fixes #17262.

Change-Id: Idfb2343e90771775e51a66c63760f458737a288c
Reviewed-on: https://go-review.googlesource.com/33730
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>