code: function parseSequenceRecord(buffer: ArrayBuffer, fileOffset: number): SequenceRecord {
    var jb = new jBinary(buffer, twoBitTypes.TYPE_SET);
    var header = jb.read('SequenceRecord');
  
    var dnaOffset = jb.tell() + 8 * header.maskBlockCount + 4;
  
    return {
      numBases: header.dnaSize,
      unknownBlockStarts: header.nBlockStarts,
      unknownBlockLengths: header.nBlockSizes,
      numMaskBlocks: header.maskBlockCount,
      maskBlockStarts: [],
      maskBlockLengths: [],
      dnaOffsetFromHeader: dnaOffset,
      offset: fileOffset
    };
  }
docs: Parses a single SequenceRecord from the start of the ArrayBuffer.
  fileOffset is the position of this sequence within the 2bit file.