user/user.go
func LoginURL(c context.Context, dest string) (string, error) {
	return LoginURLFederated(c, dest, "")
}
func LoginURL(c context.Context, dest string) (string, error) {
	return LoginURLFederated(c, dest, "")
}
// LoginURL returns a URL that, when visited, prompts the user to sign in,
// then redirects the user to the URL specified by dest.