isOpen() {
    // NOTE: Since `http.Server.listening` was added in v5.7.0
    // and we still need to support Node v4,
    // we are going to fallback to 'this._open'.
    // The following LOC would suffice for newer versions of Node.js
    // return this._webServer.listening;
    return this._open;
  }
Return `true` if server is listening. Otherwise, `false`.