anchor.diff:      for (var path in input.keys) {
         newFile(path, content: input[path]);
       }
  -    var listener = new TestMigrationListener();
  +    var listener = new _TestMigrationListener();
       var migration = NullabilityMigration(listener,
  -        permissive: usePermissiveMode, assumptions: assumptions);
  +        permissive: _usePermissiveMode, assumptions: assumptions);
       for (var path in input.keys) {
         migration.prepareInput(await session.getResolvedUnit(path));
       }
  
positive.code:   }
  }
  
  class TestMigrationListener implements NullabilityMigrationListener {
    final fixes = <SingleNullabilityFix>[];
  
    @override
    void addFix(SingleNullabilityFix fix) {
      fixes.add(fix);
    }
  }
  
negative.code:     }
      migration.finish();
      var sourceEdits = <String, List<SourceEdit>>{};
      for (var fix in listener._fixes) {
        var path = fix.source.fullName;
        expect(expectedOutput.keys, contains(path));
        (sourceEdits[path] ??= []).addAll(fix.sourceEdits);
  