I decided not to expand the API for this per discusion on #16220.

Fixes #16220

Change-Id: I65cb2eacd4ec28c79438a8f7c30024524a484ce6
Reviewed-on: https://go-review.googlesource.com/30082Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>