You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: identity.go
    path of file: ./repos/go-in-5-minutes/episode7/vendor/google.golang.org/appengine
    
    the code of the file until where you have to start completion: // Copyright 2011 Google Inc. All rights reserved.
// Use of this source code is governed by the Apache 2.0
// license that can be found in the LICENSE file.

package appengine

import (
	"time"

	"golang.org/x/net/context"

	"google.golang.org/appengine/internal
    
func AccessToken(c context.Context, scopes ...string) (token string, expiry time.Time, err error) {
	req := &pb.GetAccessTokenRequest{Scope: scopes}
	res := &pb.GetAccessTokenResponse{}

	err = internal.Call(c, "app_identity_service", "GetAccessToken", req, res)
	if err != nil {
		return "", time.Time{}, err
	}
	return res.GetAccessToken(), time.Unix(res.GetExpirationTime(), 0), nil
}