Support Page.{Alert,Confirm,Prompt,BeforeUnload} events

This patch adds Page events to fire when javascript dialogs
get opened.

lib/Page.js

@@ -60,7 +60,7 @@ class Page extends EventEmitter {
         client.on('Network.responseReceived', event => this.emit(Page.Events.ResponseReceived, event.response));
         client.on('Network.loadingFailed', event => this.emit(Page.Events.ResourceLoadingFailed, event));
         client.on('Runtime.consoleAPICalled', event => this._onConsoleAPI(event));
-        client.on('Page.javascriptDialogOpening', dialog => this.emit(Page.Events.Dialog, dialog));
+        client.on('Page.javascriptDialogOpening', event => this._onDialog(event));
         client.on('Runtime.exceptionThrown', exception => this._handleException(exception.exceptionDetails));
     }
 
@@ -207,6 +207,20 @@ class Page extends EventEmitter {
         this.emit(Page.Events.ConsoleMessage, values.join(' '));
     }
 
+    _onDialog(event) {
+        var eventType = null;
+        if (event.type === 'alert')
+            eventType = Page.Events.Alert;
+        else if (event.type === 'confirm')
+            eventType = Page.Events.Confirm;
+        else if (event.type === 'prompt')
+            eventType = Page.Events.Prompt;
+        else if (event.type === 'beforeunload')
+            eventType = Page.Events.BeforeUnload;
+        if (eventType)
+            this.emit(eventType, event.message);
+    }
+
     /**
      * @param {boolean} accept
      * @param {string} promptText
@@ -416,9 +430,12 @@ Page.ScreenshotTypes = {
 };
 
 Page.Events = {
+    Alert: 'alert',
+    BeforeUnload: 'beforeunload',
+    Confirm: 'confirm',
     ConsoleMessage: 'consolemessage',
-    Dialog: 'dialog',
     Exception: 'exception',
+    Prompt: 'prompt',
     ResourceLoadingFailed: 'resourceloadingfailed',
     ResponseReceived: 'responsereceived',
 };