function getUserByPhoneNumber(phone, onComplete) {
      return new Promise(function (resolve, reject) {
        const result = _.find(self, self._auth.users, item => {
          return item.phoneNumber === phone
        })
        if (result) {
          if (onComplete) {
            onComplete(null, result.clone())
          }
          resolve(result.clone())
        } else {
          const err = new Error('auth/user-not-found')
          if (onComplete) {
            onComplete(err, null)
          }
          reject(err)
        }
      })
    }

   function listUsers(limit, token = 0) {
      return new Promise(function (resolve) {
        const users = self_auth.users.slice(token, token + limit)

        // Return list of user records and the next page token if available.
        const result = {
          users,
          pageToken: (token + limit + 1) < self._auth.users.length ? token + 1 : null
        }

        if (!result.pageToken) {
          delete result.pageToken
        }

        resolve(result)
      })
  }