code: function loadByID(id) {
      var iconConfig = config[id];
      return loadByURL(iconConfig.url).then(function(icon) {
        return new Icon(icon, iconConfig);
      });
    }
docs: Lookup the configuration in the registry, if !registered throw an error
  otherwise load the icon [on-demand] using the registered URL.
  @param {string} id icon registry id
  @returns {angular.$q.Promise}