anchor.diff:    test('PartBuilder includes no header when `header` is empty', () async {
       await testBuilder(
           new PartBuilder([const UnformattedCodeGenerator()], header: ''),
  -        {'$pkgName|lib/a.dart': 'library a; part "a.part.dart";'},
  -        generateFor: new Set.from(['$pkgName|lib/a.dart']),
  -        outputs: {
  -          '$pkgName|lib/a.g.dart': decodedMatches(startsWith('part of')),
  +        {'$_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('part of')),
           });
     });
   
  
positive.code:     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: {
            '$pkgName|lib/a.g.dart':
                decodedMatches(startsWith('$_customHeader\npart of')),
          });
    });
  
negative.code:     await testBuilder(
          new PartBuilder([const UnformattedCodeGenerator()],
              formatOutput: (s) => s),
          {'$_pkgName|lib/a.dart': 'library a; part "a.part.dart";'},
          generateFor: new Set.from(['$_pkgName|lib/a.dart']),
          outputs: {
            '$_pkgName|lib/a.g.dart': decodedMatches(
                contains(UnformattedCodeGenerator.unformattedCode)),
          });
    });
  