This is a follow-up for https://golang.org/cl/24340.

Updates #14664.
Fixes #18374.

Change-Id: I2831556a9014d30ec70d5f91943d18c33db5b390
Reviewed-on: https://go-review.googlesource.com/34630Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>