- ignore them, if they don't.
- added tests

Fixes #18393.

Change-Id: I13f87b81ac6b9138ab5031bb3dd6bebc4c548156
Reviewed-on: https://go-review.googlesource.com/37020
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>