anchor.diff:      }
       return _convertNullability(
           _dartTypes.typeVariableType(elementMap.getTypeVariable(node.parameter)),
  -        node.nullability);
  +        node);
     }
   
     @override
  
positive.code:   DartType visitInterfaceType(ir.InterfaceType node) {
      ClassEntity cls = elementMap.getClass(node.classNode);
      return _convertNullability(
          _dartTypes.interfaceType(cls, visitTypes(node.typeArguments)),
          node.nullability);
    }
  
    @override
    DartType visitFutureOrType(ir.FutureOrType node) {
      return _convertNullability(
          _dartTypes.futureOrType(visitType(node.typeArgument)),
          node.declaredNullability);
    }
  
    @override
  
negative.code:             .toSet(),
          node.namedParameters.map((n) => visitType(n.type)).toList(),
          typeVariables ?? const <FunctionTypeVariable>[]);
      DartType type = _convertNullability(functionType, node);
      for (ir.TypeParameter typeParameter in node.typeParameters) {
        currentFunctionTypeParameters.remove(typeParameter);
      }
  