code: function _checkForUrlProperty(uri, { url, customUrl }) {
    if (!url && !customUrl) {
      return bluebird.reject(new Error('Page does not have a `url` or `customUrl` property set'));
    }
  
    return bluebird.resolve(url || customUrl);
  }
docs: Return the url for a page based on own url. This occurs when a page
  falls outside of all publishing rules supplied by the implementation
  BUT there is a `url` property already set on the page data.
  
  @param {String} uri
  @param {Object} pageData
  @param {String} pageData.url
  @param {String} pageData.customUrl
  @returns {Promise}