This is an initial implementation.
There are many rough edges and TODOs,
which will hopefully be polished out
with use.

Fixes #12071.

Change-Id: I1d6fd5a343063b5200623bceef2c2cfcc885794e
Reviewed-on: https://go-review.googlesource.com/13472Reviewed-by: Keith Randall <khr@golang.org>