CL 4310 introduced these functions, but their
implementation does not match with their published
documentation. Correct the implementation.

Change-Id: I285e41f9c7c5fc4e550ff59b0adb8b2bcbf6737a
Reviewed-on: https://go-review.googlesource.com/17997Reviewed-by: Yasuhiro MATSUMOTO <mattn.jp@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>