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