[FIX] web_drop_target: remove ES6 sintax

web_drop_target/static/src/js/web_drop_target.js

@@ -107,7 +107,7 @@ odoo.define('web_drop_target', function(require) {
             });
         },
 
-        _add_overlay() {
+        _add_overlay: function() {
             if(!this._drop_overlay){
                 var o_content = jQuery('.o_content'),
                     view_manager = jQuery('.o_view_manager_content');
@@ -125,7 +125,7 @@ odoo.define('web_drop_target', function(require) {
             }
         },
         
-        _remove_overlay() {
+        _remove_overlay: function() {
             this._drop_overlay.remove();
             this._drop_overlay = null;
         },