use a flow type to pass config data through a Promise chain

example.js

  type RunConfig = {
    runner: ExtensionRunner,
    profile: FirefoxProfile,
    client?: RemoteFirefox,
    addonId?: string,
    firefox?: FirefoxProcess,
  };
  
  ...
  
  .then(({firefox, profile, client, addonId}: RunConfig) => {
      if (!client) {
        throw new Error('RemoteFirefox `client` should not be missing here');
      }
      if (!addonId) {
        throw new Error('addonId should not be missing here');
      }
      if (!firefox) {
        throw new Error('FirefoxProcess `firefox` should not be missing here');
      }

      if (noReload) {
        log.debug('Extension auto-reloading has been disabled');
      } else {
        log.debug(
          `Reloading extension when the source changes; id=${addonId}`);
        reloadStrategy({
          firefox, profile, client, sourceDir, artifactsDir, addonId,
        });
      }
      return firefox;
    });