anchor.diff:      await testBuilder(
           new PartBuilder([const UnformattedCodeGenerator()],
               header: _customHeader),
  -        {'$pkgName|lib/a.dart': 'library a; part "a.part.dart";'},
  -        generateFor: new Set.from(['$pkgName|lib/a.dart']),
  -        outputs: {
  -          '$pkgName|lib/a.g.dart':
  +        {'$_pkgName|lib/a.dart': 'library a; part "a.part.dart";'},
  +        generateFor: new Set.from(['$_pkgName|lib/a.dart']),
  +        outputs: {
  +          '$_pkgName|lib/a.g.dart':
                 decodedMatches(startsWith('$_customHeader\npart of')),
           });
     });
  
positive.code: 
  final _customHeader = '// Copyright 1979';
  
  const pkgName = 'pkg';
  
  const _testLibContent = r'''
  library test_lib;
  
negative.code:   });
  
    test('Allow no "library"  by default', () async {
      var sources = _createPackageStub(testLibContent: 'class A {}');
      var builder = new PartBuilder([const CommentGenerator()]);
  
      await testBuilder(builder, sources,
          outputs: {'$_pkgName|lib/test_lib.g.dart': _testGenNoLibrary});
    });
  
    test('Does not fail when there is no output', () async {
      var sources = _createPackageStub(testLibContent: 'class A {}');
      var builder = new PartBuilder([const CommentGenerator(forClasses: false)]);
      await testBuilder(builder, sources, outputs: {});
    });
  
    test('Use new part syntax when no library directive exists', () async {
      var sources = _createPackageStub(testLibContent: 'class A {}');
      var builder = new PartBuilder([const CommentGenerator()]);
      await testBuilder(builder, sources,
          outputs: {'$_pkgName|lib/test_lib.g.dart': _testGenNoLibrary});
    });
  
    test(
  