+ test.

Fixes #16230

Change-Id: Idac995437146a9df9e73f094d2a31abc25b1fa62
Reviewed-on: https://go-review.googlesource.com/24681Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>