low it.
   *
   * @constructor
   * @extends {tr.ui.tracks.Track}
   */
  const SpacingTrack = tr.ui.b.define('spacing-track', tr.ui.tracks.Track);

  SpacingTrack.prototype = {
    __proto__: tr.ui.tracks.Track.prototype,

    decorate(viewport) {
      tr.ui.tracks.Track.prototype.decorate.call(this, viewport);
      Polymer.dom(this).classList.add('spacing-track');

      this.heading_ = document.createElement('tr-ui-b-heading');
      Polymer.dom(this).appendChild(this.heading_);
    },

    addAllEvent