anchor.diff:      packagesPath: argResults[_kOptionPackages],
       includeRobotoFonts: true,
     );
  -  if (result != 0) {
  -    printError('Error building $outputPath: $result.');
  +  if (buildResult != 0) {
  +    printError('Error building $outputPath: $buildResult.');
  +    exit(buildResult);
     }
  -  exit(result);
  +  final int headerResult = _addHeader(outputPath, argResults[_kOptionHeader]);
  +  if (headerResult != 0) {
  +    printError('Error adding header to $outputPath: $headerResult.');
  +  }
  +  exit(headerResult);
   }
  +
  +int _addHeader(String outputPath, String header) {
  +  try {
  +    final File outputFile = new File(outputPath);
  +    final List<int> content = outputFile.readAsBytesSync();
  +    outputFile.writeAsStringSync('$header\n');
  +    outputFile.writeAsBytesSync(content, mode: FileMode.APPEND);
  +    return 0;
  +  } catch (_) {
  +    return 1;
  +  }
  +}
  
positive.code:   _kOptionTarget,
    _kOptionPackages,
    _kOptionOutput,
    _kOptionSnapshot,
    _kOptionDepfile,
    _kOptionWorking,
  
negative.code:     ..addOption(_kOptionTarget, help: 'The entry point into the app')
      ..addOption(_kOptionPackages, help: 'The .packages file')
      ..addOption(_kOptionOutput, help: 'The generated flx file')
      ..addOption(_kOptionHeader, help: 'The header of the flx file')
      ..addOption(_kOptionSnapshot, help: 'The generated snapshot file')
      ..addOption(_kOptionDepfile, help: 'The generated dependency file')
      ..addOption(_kOptionWorking,
  