This is basically a mini-bootstrap, to reach a fixed point.

Change-Id: I88abad3d3ac961c3d11a48cb64d625d458684ef7
Reviewed-on: https://go-review.googlesource.com/74792
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>