anchor.diff:    String url = args['hosted-url'];
     String footerFilePath = _resolveTildePath(args['footer']);
     if (footerFilePath != null && !new File(footerFilePath).existsSync()) {
  -    print(
  -        "Warning: unable to locate the file with footer at ${footerFilePath}.");
  +    print("Error: unable to locate the file with footer at ${footerFilePath}.");
       exit(1);
     }
     String headerFilePath = _resolveTildePath(args['header']);
     if (headerFilePath != null && !new File(headerFilePath).existsSync()) {
  -    print(
  -        "Warning: unable to locate the file with header at ${headerFilePath}.");
  +    print("Error: unable to locate the file with header at ${headerFilePath}.");
       exit(1);
     }
   
  
positive.code: 
    Directory sdkDir = cli_util.getSdkDir(arguments);
    if (sdkDir == null) {
      print("Warning: unable to locate the Dart SDK. Please use the --dart-sdk "
          "command line option or set the DART_SDK environment variable.");
      exit(1);
    }
  
negative.code:   Directory packageRootDir;
    if (args['package-root'] != null) {
      packageRootDir = new Directory(_resolveTildePath(args['package-root']));
    }
  
    if (args.rest.isNotEmpty) {
      var unknownArgs = args.rest.join(' ');
      print('Error: detected unknown command-line argument(s): $unknownArgs');
      _printUsageAndExit(parser, exitCode: 1);
    }
  
    PackageMeta packageMeta = sdkDocs
  