Noticed while working on issue 30488.

Change-Id: Ia3655e07c939d03925b3560eeba24c60e24c136c
Reviewed-on: https://go-review.googlesource.com/c/go/+/175917
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>