anchor.diff:          'mock-sdk', abbr: 'm', help: 'Use a mock Dart SDK', defaultsTo: false)
       ..addFlag('new-checker', abbr: 'n', help: 'Use the new type checker',
           defaultsTo: false)
  -    ..addOption('out', abbr: 'o', help: 'Output directory', defaultsTo: null);
  -  return parser.parse(argv);
  +    ..addOption('out', abbr: 'o', help: 'Output directory', defaultsTo: null)
  +    ..addFlag('help', abbr: 'h', help: 'Display this message');
  +
  +void _showUsageAndExit() {
  +  print('usage: dartdevc [<options>] <file.dart>\n');
  +  print('<file.dart> is a single Dart file to process.\n');
  +  print('<options> include:\n');
  +  print(argParser.usage);
  +  exit(1);
   }
   
  -void main(List argv) {
  -  ArgResults args = parse(argv);
  +int main(List<String> argv) {
  +  ArgResults args = argParser.parse(argv);
  +  if (args['help']) _showUsageAndExit();
  +
     bool shouldMockSdk = args['mock-sdk'];
     String dartSdkPath;
     if (!shouldMockSdk) {
  
positive.code: 
    if (args.rest.length == 0) {
      print('Expected filename.');
      exit(1);
    }
  
    String levelName = args['log'].toUpperCase();
  
negative.code: #!/usr/bin/env dart
  
  /// Command line tool to run the checker on a Dart program.
  library ddc.bin.checker;
  
  