Updates golang/go#14959

Change-Id: Ib91c359c3df919df0b30e584d38e56f79f3e3dc9
Reviewed-on: https://go-review.googlesource.com/21388Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>