lib/Assertion.php
function instanceOf(string $className): self {
    return $this->expect($this->target, isInstanceOf($className));
  }
function instanceOf(string $className): self {
    return $this->expect($this->target, isInstanceOf($className));
  }
Reports an error if the target is not an instance of the specified class.
@param string $className The name of the class to test.
@return self This instance.