encode method Null safety

Implementation

List<int>? encode(dynamic data, {int? level, dynamic output}) {
  dynamic outputStream = output ?? OutputStream();

  // The GZip format has the following structure:
  // Offset   Length   Contents
  // 0      2 bytes  magic header  0x1f, 0x8b (\037 \213)
  // 2      1 byte   compression method
  //                  0: store (copied)
  //                  1: compress
  //                  2: pack
  //                  3: lzh
  //                  4..7: reserved
  //                  8: deflate
  // 3      1 byte   flags
  //                  bit 0 set: file probably ascii text
  //                  bit 1 set: continuation of multi-part gzip file, part number present
  //                  bit 2 set: extra field present
  //                  bit 3 set: original file name present
  //                  bit 4 set: file comment present
  //                  bit 5 set: file is encrypted, encryption header present
  //                  bit 6,7:   reserved
  // 4      4 bytes  file modification time in Unix format
  // 8      1 byte   extra flags (depend on compression method)
  // 9      1 byte   OS type
  // [
  //        2 bytes  optional part number (second part=1)
  // ]?
  // [
  //        2 bytes  optional extra field length (e)
  //       (e)bytes  optional extra field
  // ]?
  // [
  //          bytes  optional original file name, zero terminated
  // ]?
  // [
  //          bytes  optional file comment, zero terminated
  // ]?
  // [
  //       12 bytes  optional encryption header
  // ]?
  //          bytes  compressed data
  //        4 bytes  crc32
  //        4 bytes  uncompressed input size modulo 2^32

  outputStream.writeUint16(SIGNATURE);
  outputStream.writeByte(DEFLATE);

  var flags = 0;
  var fileModTime = DateTime.now().millisecondsSinceEpoch ~/ 1000;
  var extraFlags = 0;
  var osType = OS_UNKNOWN;

  outputStream.writeByte(flags);
  outputStream.writeUint32(fileModTime);
  outputStream.writeByte(extraFlags);
  outputStream.writeByte(osType);

  Deflate deflate;
  if (data is List<int>) {
    deflate = Deflate(data, level: level, output: outputStream);
  } else {
    deflate = Deflate.buffer(data as InputStreamBase,
        level: level, output: outputStream);
  }

  if (outputStream is! OutputStream) {
    deflate.finish();
  }

  outputStream.writeUint32(deflate.crc32);

  outputStream.writeUint32(data.length);

  if (outputStream is OutputStreamBase) {
    outputStream.flush();
  }

  if (outputStream is OutputStream) {
    return outputStream.getBytes();
  }
  return null;
}