dowidth is fundamentally unsafe to call from the back end;
it will cause data races.

Replace all calls to dowidth in the backend with
assertions that the width has been calculated.

Then fix all the cases in which that was not so,
including the cases from #20145.

Fixes #20145.

Change-Id: Idba3d19d75638851a30ec2ebcdb703c19da3e92b
Reviewed-on: https://go-review.googlesource.com/41970
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>