It was using a mix of stdout and stderr. Most users won't notice, but
it's inconsistent for no apparent reason. In particular, I noticed as
some of my tool execution tests started failing.
Change-Id: I9afe5f5bed0a575d3ba20e8dc1cc593c35565cf9
Reviewed-on: https://go-review.googlesource.com/c/162717
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>