This was accidentally broken by CL 127755.

Fixes #29333

Change-Id: I5e92048c64a55c1699d6c38eb4dbbd51c817b820
Reviewed-on: https://go-review.googlesource.com/c/155037
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>