inside class.admin.php , line 375 inside print_footer() there are only replacements for:
public function print_footer($activateJsAdmin = false) {
...
$footer_template->set_var(array(
'IS_ADMIN' => ($this->get_user_id()==1),
'BACKEND_BODY_MODULE_JS' => $this->register_backend_modfiles_body('js'),
'WB_URL' => WB_URL,
'ADMIN_URL' => ADMIN_URL,
'THEME_URL' => THEME_URL,
'HELPER_URL' => WB_URL.'/framework/helpers',
) );
....
}
So the BACKEND_BODY_MODULE _JS only displays backend_body.js and there is no placeholder to display backend.js