[DevTools] Typed events and event listeners.

third_party/WebKit/Source/devtools/front_end/ui/DropDownMenu.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 /**
  * @unrestricted
  */
-UI.DropDownMenu = class extends Common.Object {
+UI.DropDownMenu = class extends Common.ObjectBase {
   /**
    * @param {!Element} element
    */
   constructor(element) {
     super();
     /** @type {!Array.<!UI.DropDownMenu.Item>} */
     this._items = [];
 
     element.addEventListener('mousedown', this._onMouseDown.bind(this));
   }
 
   /**
    * @param {!Event} event
    */
   _onMouseDown(event) {
     if (event.which !== 1)
       return;
     var menu = new UI.ContextMenu(event);
     for (var item of this._items)
       menu.appendCheckboxItem(item.title, this._itemHandler.bind(this, item.id), item.id === this._selectedItemId);
     menu.show();
   }
 
   /**
    * @param {string} id
    */
   _itemHandler(id) {
-    this.dispatchEventToListeners(UI.DropDownMenu.Events.ItemSelected, id);
+    this.dispatchEventToListeners(new UI.DropDownMenu.ItemSelectedEvent(id));
   }
 
   /**
    * @param {string} id
    * @param {string} title
    */
   addItem(id, title) {
     this._items.push({id: id, title: title});
   }
 
   /**
    * @param {string} id
    */
   selectItem(id) {
     this._selectedItemId = id;
   }
 
   clear() {
     this._items = [];
     delete this._selectedItemId;
   }
 };
 
 /** @typedef {{id: string, title: string}} */
 UI.DropDownMenu.Item;
 
-/** @enum {symbol} */
-UI.DropDownMenu.Events = {
-  ItemSelected: Symbol('ItemSelected')
+UI.DropDownMenu.ItemSelectedEvent = class {
+  /**
+   * @param {string} id
+   */
+  constructor(id) {
+    this.id = id;
+  }
 };