function getIcon(id) {
    id = id || '';

    // If the "id" provided is not a string, the only other valid value is a $sce trust wrapper
    // over a URL string. If the value is not trusted, this will intentionally throw an error
    // because the user is attempted to use an unsafe URL, potentially opening themselves up
    // to an XSS attack.
    if (!angular.isString(id)) {
      id = $sce.getTrustedUrl(id);
    }

    // If already loaded and cached, use a clone of the cached icon.
    // Otherwise either load by URL, or lookup in the registry and then load by URL, and cache.

    if (iconCache[id]) {
      return $q.when(transformClone(iconCache[id]));
    }

    if (urlRegex.test(id) || dataUrlRegex.test(id)) {
      return loadByURL(id).then(cacheIcon(id));
    }

    if (id.indexOf(':') === -1) {
      id = '$default:' + id;
    }

    var load = config[id] ? loadByID : loadFromIconSet;
    return load(id)
      .then(cacheIcon(id));
  }
Actual $mdIcon service is essentially a lookup function
@param {*} id $sce trust wrapper over a URL string, URL, icon registry id, or icon set id
@returns {angular.$q.Promise}