static bufferEnd(b) {
    if (!b) {
      return null;
    }
    if (shaka.media.TimeRangesUtils.isBufferNegligible_(b)) {
      return null;
    }
    return b.length ? b.end(b.length - 1) : null;
  }
Gets the last timestamp in the buffer.

@param {TimeRanges} b
@return {?number} The last buffered timestamp, in seconds, if |buffered|
  is non-empty; otherwise, return null.