public OAuthSessionFactory(SlackApi slackApi) {
    this.slackApi = slackApi;
  }

  public void start() throws Exception {
    if (mockWebServer != null) throw new IllegalStateException();

    mockWebServer = new MockWebServer();
    mockWebServer.setDispatcher(this);
    mockWebServer.start(slackApi.port);
  }

  public HttpUrl newAuthorizeUrl(String scopes, String team, Listener listener) {