anchor.diff:        return null;
       }
       if (results[HELP_OPTION]) {
  +      printUsage(parser);
  +      return null;
  +    }
  +    sdkPath = results[DART_SDK_OPTION];
  +    if (sdkPath is! String) {
  +      print('Missing path to Dart SDK');
  +      printUsage(parser);
  +      return null;
  +    }
  +    Directory sdkDir = new Directory(sdkPath);
  +    if (!sdkDir.existsSync()) {
  +      print('Specified Dart SDK does not exist: $sdkPath');
         printUsage(parser);
         return null;
       }
  
positive.code:   static const BINARY_NAME = 'dartdeps';
  
    /**
     * The name of the option used to print usage information.
     */
    static const String HELP_OPTION = 'help';
  
negative.code:    * The command line arguments.
     */
    final List<String> args;
  
    /**
     * The path to the Dart SDK used during analysis.
     */
    String sdkPath;
  
    /**
     * The manager for the analysis server.
  