olymer
     * @appliesMixin Polymer.PropertyAccessors
     * @memberof Polymer
     */
    Polymer.DirMixin = Polymer.dedupingMixin((base) => {

      if (!observer) {
        getRTL();
        observer = new MutationObserver(updateDirection);
        observer.observe(document.documentElement, {attributes: true, attributeFilter: ['dir']});
      }

      /**
       * @constructor
       * @extends {base}
       * @implements {Polymer_PropertyAccessors}
       */
      const elementBase = Polymer.PropertyAcc