Based in part on khr's CL 2500.

Updates #17725
Updates #18121

Change-Id: I744e1f92fc2104e6c5bd883a898c30b2eea8cc31
Reviewed-on: https://go-review.googlesource.com/35555
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>