function detectEncoding(label, input_stream) {
    if (input_stream.match([0xFF, 0xFE])) {
      input_stream.offset(2);
      return 'utf-16le';
    }
    if (input_stream.match([0xFE, 0xFF])) {
      input_stream.offset(2);
      return 'utf-16be';
    }
    if (input_stream.match([0xEF, 0xBB, 0xBF])) {
      input_stream.offset(3);
      return 'utf-8';
    }
    return label;
  }
@param {string} label The encoding label.
@param {ByteInputStream} input_stream The byte stream to test.