static from6ArgsTextTrackCue_(startTime, endTime, text) {
    const id = startTime + '-' + endTime + '-' + text;
    // Quoting the access to the TextTrackCue object to satisfy the compiler.
    return new window['TextTrackCue'](id, startTime, endTime, text);
  }
Draft spec TextTrackCue with 6 constructor arguments (5th & 6th are
optional).
@see {@link https://bit.ly/2KaGSP2 W3C Working Draft 29 March 2012}.

@param {number} startTime
@param {number} endTime
@param {string} text
@return {!TextTrackCue}
@private