This was supposed to be in CL 31354
but was dropped due to a Git usage error.

For #16573.

Change-Id: I3d99087c8efc8cbc016c55e8365d0005f79d1b2f
Reviewed-on: https://go-review.googlesource.com/31461Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>