It is going to be used by a new tool.
Moved to an internal package so it does not become a publicly supported
api.
Modified the tests so they don't depend on the fix_test infrastructure.
Change-Id: Ib8ebef24dc23e180960af04aa3d06b5f41a7c02b
Reviewed-on: https://go-review.googlesource.com/99678
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
