lithium issue with custom form helper not working

gistfile1.aw

<?php

namespace app\extensions\helper;

class Form extends \lithium\template\helper\Form {
  // Add or override Form helper methods
  public function field() {
    var_dump('bar');
    die();
  }
}
?>

//in my view:

<?=$this->form->create($object); ?>
<?=$this->form->field('title');?> 
<?=$this->form->end(); ?>