This was disallowed for error-checking reasons but people ask for
it, it's easy, and it's clear what it all means.

Fixes #7323.

Change-Id: I26542f5ac6519e45b335ad789713a4d9e356279b
Reviewed-on: https://go-review.googlesource.com/9537Reviewed-by: Russ Cox <rsc@golang.org>