This was a regression from 1.7. See the issue for details.

Fixes #18149.

Change-Id: Ic8f5a35d14edf9254b1275400316cff7aff32a27
Reviewed-on: https://go-review.googlesource.com/33799
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>