code: private function parseDirective($isConst)
      {
          $start = $this->lexer->token;
          $this->expect(Token::AT);
  
          return new DirectiveNode([
              'name'      => $this->parseName(),
              'arguments' => $this->parseArguments($isConst),
              'loc'       => $this->loc($start),
          ]);
      }
docs: @param bool $isConst
  
  @return DirectiveNode
  
  @throws SyntaxError