anchor.diff:  }
   
   @reflectiveTest
  -class IsPotentiallyNullableTest extends _NullableBase {
  +class IsPotentiallyNullableTest extends AbstractTypeSystemNullSafetyTest {
     void isNotPotentiallyNullable(DartType type) {
       expect(typeSystem.isPotentiallyNullable(type), isFalse);
     }
  
positive.code:     expect(actual.nullabilitySuffix, NullabilitySuffix.none);
    }
  }
  
  abstract class _NullableBase with ElementsTypesMixin {
    @override
    TypeProvider typeProvider;
  
    TypeSystemImpl typeSystem;
  
    FeatureSet get testFeatureSet {
      return FeatureSet.forTesting();
    }
  
    void setUp() {
      var analysisContext = TestAnalysisContext(
        featureSet: testFeatureSet,
      );
      typeProvider = analysisContext.typeProviderLegacy;
      typeSystem = analysisContext.typeSystemLegacy;
    }
  }
  
negative.code: }
  
  @reflectiveTest
  class PromoteToNonNullTest extends AbstractTypeSystemNullSafetyTest {
    test_dynamic() {
      _check(dynamicType, dynamicType);
    }
  