code: function syncAddressBook(addressBook, options) {
    options.basicSync = basicSync;
    options.webdavSync = webdavSync;
    return webdav.syncCollection(addressBook, options);
  }
docs: @param {dav.Calendar} calendar the calendar to fetch updates to.
  @return {Promise} promise will resolve with updated calendar object.
  
  Options:
  
  (dav.Sandbox) sandbox - optional request sandbox.
  (String) syncMethod - either 'basic' or 'webdav'. If unspecified, will
  try to do webdav sync and failover to basic sync if rfc 6578 is not
  supported by the server.
  (dav.Transport) xhr - request sender.