For #14962.

Change-Id: I3539d882487c99dee99ac953e039b79c6b963cf9
Reviewed-on: https://go-review.googlesource.com/21150Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>