anchor.diff:  }
   ''');
   
  -    assertConstraint(
  -        [ConstraintVariable.always],
  -        _either(decoratedTypeAnnotation('int').node.nullable,
  -            checkExpression('null').nullCheck));
  +    assertNullCheck(checkExpression('null'), NullabilityNode.always,
  +        contextNode: decoratedTypeAnnotation('int').node);
     }
   
     test_stringLiteral() async {
  
positive.code: }
  ''');
  
      assertConstraint([decoratedTypeAnnotation('C c').node.nullable],
          checkExpression('c.m').nullCheck);
    }
  
    test_methodInvocation_target_demonstrates_non_null_intent() async {
  
negative.code: int f(int i, int j) => i + j;
  ''');
  
      assertNullCheck(
          checkExpression('i +'), decoratedTypeAnnotation('int i').node);
    }
  
    test_binaryExpression_add_left_check_custom() async {
  