I can't reproduce the user's bug yet, but this might fix or at least
help clarify what's happening.

Also deflakes a test.

Updates golang/go#13932

Change-Id: If56bdd833f183d4502701e65e56749434bd82150
Reviewed-on: https://go-review.googlesource.com/18576Reviewed-by: Andrew Gerrand <adg@golang.org>