static bufferStart(b) {
    if (!b) {
      return null;
    }
    if (shaka.media.TimeRangesUtils.isBufferNegligible_(b)) {
      return null;
    }
    // Workaround Edge bug: https://bit.ly/2JYLPeB
    if (b.length == 1 && b.start(0) < 0) {
      return 0;
    }
    return b.length ? b.start(0) : null;
  }
Gets the first timestamp in the buffer.

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