Introduced in CLs 29380 and 30011.

Change-Id: I3d3641e8748ce0adb57b087a1fcd62f295ade665
Reviewed-on: https://go-review.googlesource.com/31933
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>