This still depends on Curfn, but it's progress.

Updates #15756

Change-Id: Ic32fe56f44fcfbc023e7668d4dee07f8b47bf3a4
Reviewed-on: https://go-review.googlesource.com/26661
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>