Implemented event-dispatching for button clicks.

ui/login/screen.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.
 
 /**
  * @fileoverview Base class for all login WebUI screens.
  */
 cr.define('login', function() {
+  /** @const */ var CALLBACK_BUTTON_CLICKED = 'buttonClicked';
+  /** @const */ var CALLBACK_CONTEXT_CHANGED = 'contextChanged';
   /** @const */ var CALLBACK_USER_ACTED = 'userActed';
-  /** @const */ var CALLBACK_CONTEXT_CHANGED = 'contextChanged';
 
   function doNothing() {};
 
   var querySelectorAll = HTMLDivElement.prototype.querySelectorAll;
 
   var Screen = function(sendPrefix) {
     this.sendPrefix_ = sendPrefix;
     this.screenContext_ = null;
     this.contextObservers_ = {};
   };
@@ skipping 69 matching lines @@
     },
 
     /**
      * @final
      */
     commitContextChanges: function() {
       return this.commitContextChangesImpl_.apply(this, arguments);
     },
 
     /**
+     * @final
+     */
+    declareButton: function(id) {
+      var self = this;
+      var button = this.ownerDocument.createElement('button');
+      button.id = id;
+
+      button.addEventListener('click', function(e) {
+        self.sendImpl_(CALLBACK_BUTTON_CLICKED, id);
+        e.stopPropagation();
+      });
+
+      return button;
+    },
+
+    /**
      * @override
      * @final
      */
     querySelectorAll: function() {
       return this.querySelectorAllImpl_.apply(this, arguments);
     },
 
     /**
      * Does the following things:
      *  * Creates screen context.
@@ skipping 217 matching lines @@
 
       cr.define('login', function() {
         var result = {};
         result[name] = api;
         return result;
       });
     }
   };
 });