Mostly complete but a few TODOs remain for future CLs.

For #15810.

Change-Id: I81ee19d1088d192cf709a5f7e6b7bcc44ad892ac
Reviewed-on: https://go-review.googlesource.com/23379Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>