anchor.diff:            '$_pkgName|lib/a.g.dart': decodedMatches(
               '''
   // GENERATED CODE - DO NOT MODIFY BY HAND
  -// @dart=2.9
  +// @dart=2.12
   
   part of a;
   
  
positive.code:         const CombiningBuilder(),
          {
            '$_pkgName|lib/a.dart': '''
  // @dart=2.9
  library a;
  part "a.g.dart";
  ''',
  
negative.code:           header: '',
          ),
          {
            '$_pkgName|lib/a.dart': '''// @dart=2.12
  part "a.foo.dart";'''
          },
          generateFor: {'$_pkgName|lib/a.dart'},
          outputs: {
            '$_pkgName|lib/a.foo.dart':
                decodedMatches(startsWith('// @dart=2.12\n')),
          },
        );
      });
  