Fixesgolang/go#21055.
Change-Id: I126cef4496c15424048e39b2af111c95580b90fb
Reviewed-on: https://go-review.googlesource.com/49390
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>