nto a provided div.
   * @constructor
   */
  const Track = tr.ui.b.define('track',
      tr.ui.b.ContainerThatDecoratesItsChildren);
  Track.prototype = {
    __proto__: tr.ui.b.ContainerThatDecoratesItsChildren.prototype,

    decorate(viewport) {
      tr.ui.b.ContainerThatDecoratesItsChildren.prototype.decorate.call(this);
      if (viewport === undefined) {
        throw new Error('viewport is required when creating a Track.');
      }

      this.viewport_ = viewport;
      Polymer.dom(this).classList