Fixes #7991.
Fixes #12719.

Change-Id: I5650fa35ec5d49addeda6cc6e7fa93cfbe1cdfc0
Reviewed-on: https://go-review.googlesource.com/17385Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>