anchor.diff:    }
   }
   
  +/// A [Builder] which generateds `part of` files.
   class PartBuilder extends _Builder {
  +  /// Wrap [generators] as a [Builder] that generates `part of` files.
  +  ///
  +  /// [generatedExtension] indicates what files will be created for each `.dart`
  +  /// input. Defaults to `.g.dart`.
  +  ///
  +  /// [formatOutput] is called to format the generated code. Defaults to
  +  /// [DartFormatter.format].
  +  ///
  +  /// May set [requireLibraryDirective] to `false` in order to opt-in to
  +  /// supporting a `1.25.0` feature of `part of` being usable without an
  +  /// explicit `library` directive. Developers should restrict their `pubspec`
  +  /// accordingly:
  +  /// ```yaml
  +  /// sdk: '>=1.25.0 <2.0.0'
  +  /// ```
     PartBuilder(List<Generator> generators,
         {OutputFormatter formatOutput,
         String generatedExtension: '.g.dart',
  
positive.code:   /// Function that determines how the generated code is formatted.
    final OutputFormatter formatOutput;
  
    /// What underlying generators are wrapped to form this [Builder].
    final List<Generator> generators;
  
    /// For a given `.dart` file what extension is used to generate code.
    ///
    /// Defaults to `.g.dart`.
    final String generatedExtension;
  
    /// Whether to emit a standalone (non-`part`) file in this builder.
    final bool isStandalone;
  
    final bool _requireLibraryDirective;
  
    /// Wrap [generators] to form a [Builder]-compatible API.
    ///
    /// May set [requireLibraryDirective] to `false` in order to opt-in to
    /// supporting a `1.25.0` feature of `part of` being usable without an
    /// explicit `library` directive. Developers should restrict their `pubspec`
    /// accordingly:
    /// ```yaml
    /// sdk: '>=1.25.0 <2.0.0'
    /// ```
    _Builder(this.generators,
        {OutputFormatter formatOutput,
        this.generatedExtension: '.g.dart',
        this.isStandalone: false,
        bool requireLibraryDirective: true})
        : formatOutput = formatOutput ?? _formatter.format,
          _requireLibraryDirective = requireLibraryDirective {
      if (generatedExtension == null) {
        throw new ArgumentError.notNull('generatedExtension');
      }
      if (generatedExtension.isEmpty || !generatedExtension.startsWith('.')) {
        throw new ArgumentError.value(generatedExtension, 'generatedExtension',
            'Extension must be in the format of .*');
      }
      if (this.isStandalone && this.generators.length > 1) {
        throw new ArgumentError(
            'A standalone file can only be generated from a single Generator.');
      }
  
negative.code:     final outputId = _generatedFile(buildStep.inputId);
  
      var contentBuffer = new StringBuffer();
      if (!_isStandalone) {
        var asset = buildStep.inputId;
        var name = nameOfPartial(
          library,
  