constructor(countries) {
    /**
     * @private {!Array<!firebaseui.auth.data.country.Country>} The list of
     *     countries to construct a prefix tree for.
     */
    this.countries_ = countries;
    /**
     * @private {
     * !goog.structs.Trie<!Array<!firebaseui.auth.data.country.Country>>} The
     *     prefix tree.
     */
    this.trie_ = new goog.structs.Trie();
    // Initialize prefix tree like structure.
    this.init_();
  }
@param {!Array<!firebaseui.auth.data.country.Country>} countries The list
    of countries to construct a prefix tree for.