anchor.diff:              formatOutput: formatOutput,
               generatedExtension: '.$partId.g.part',
               additionalOutputExtensions: additionalOutputExtensions,
  -            header: '',
  -            outputPartOf: false) {
  +            header: '') {
       if (!_partIdRegExp.hasMatch(partId)) {
         throw new ArgumentError.value(
             partId,
  
positive.code:       contentBuffer.writeln(_header);
      }
  
      if (!_isStandalone) {
        var asset = buildStep.inputId;
        var name = nameOfPartial(library, asset);
        if (name == null) {
  
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');
      }
  