The API extends the old API but is intended to be backwards
compatible. The old test file serves as a test for this purpose.

I will send a CL to update the vendored mirror in core after
1.8 is out.

Change-Id: I538df20d42c213d9e038bdb0dad76b6cc132d4fd
Reviewed-on: https://go-review.googlesource.com/34770
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>