Verified by manually enabling debug mode and running make.bash.

Fixes #20684.

Change-Id: I041f2ca6ef1d4198815724d98f61511072d63581
Reviewed-on: https://go-review.googlesource.com/45971
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>