Not much testing yet, but the test now exists.

Another step toward #13822.

Change-Id: Idb2b06bf53a6113c83008150b4c0b631bb195279
Reviewed-on: https://go-review.googlesource.com/18844Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>