anchor.diff:              formatOutput: formatOutput,
               generatedExtension: generatedExtension,
               additionalOutputExtensions: additionalOutputExtensions,
  -            isStandalone: true,
               header: header);
   }
   
  
positive.code:       throw new ArgumentError.value(_generatedExtension, 'generatedExtension',
            'Extension must be in the format of .*');
      }
      if (_isStandalone && _generators.length > 1) {
        throw new ArgumentError(
            'A standalone file can only be generated from a single Generator.');
      }
  
negative.code:       {String formatOutput(String code),
        String generatedExtension = '.g.dart',
        List<String> additionalOutputExtensions = const [],
        String header})
        : _generatedExtension = generatedExtension,
          buildExtensions = {
            '.dart': [generatedExtension]..addAll(additionalOutputExtensions)
          },
          formatOutput = formatOutput ?? _formatter.format,
          _header = (header ?? defaultFileHeader).trim() {
      if (_generatedExtension == null) {
        throw new ArgumentError.notNull('generatedExtension');
      }
  