anchor.diff:  }
   
   /// Mixin containing test cases for the provisional API.
  -mixin ProvisionalApiTestCases on ProvisionalApiTestBase {
  +mixin _ProvisionalApiTestCases on _ProvisionalApiTestBase {
     test_conditional_assert_statement_does_not_imply_non_null_intent() async {
       var content = '''
   void f(bool b, int i) {
  
positive.code:     for (var path in input.keys) {
        newFile(path, content: input[path]);
      }
      var listener = new TestMigrationListener();
      var migration = NullabilityMigration(listener,
          permissive: usePermissiveMode, assumptions: assumptions);
      for (var path in input.keys) {
        migration.prepareInput(await session.getResolvedUnit(path));
      }
  
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);
  