New in Go 1.7 so still possible to change.
This allows implementations not tied to *net.Dialer.

Fixes #15748.

Change-Id: I5fabbf13c7f1951c06587a4ccd120def488267ce
Reviewed-on: https://go-review.googlesource.com/23489Reviewed-by: Ian Lance Taylor <iant@golang.org>