constructor(testOnlyIsRightToLeft = false) {
    super();
    this.testOnlyIsRightToLeft_ = testOnlyIsRightToLeft;
    SliderBase.call(
        this, undefined /* domHelper */,
        (value) => value > 5 ? 'A big value.' : 'A small value.');
  }
@param {boolean=} testOnlyIsRightToLeft This parameter is necessary to
    tell if the slider is rendered right-to-left when creating thumbs
    (before entering the document). Used only for test purposes.