Bug 939192 - Fix about flyout js error with updater. r=bbondy

browser/metro/base/content/bindings/flyoutpanel.xml

@@ -66,6 +66,10 @@
             this.classList.remove("flyoutpanel-slide-in");
             this.setAttribute("isSlidingOut", true);
             this.removeAttribute("isSlidIn");
+
+            let event = document.createEvent("Events");
+            event.initEvent("MozFlyoutPanelHiding", true, false);
+            this.dispatchEvent(event);
           ]]>
         </body>
       </method>

browser/metro/base/content/flyoutpanels/AboutFlyoutPanel.js

@@ -39,6 +39,7 @@ let AboutFlyoutPanel = {
     }
 
     window.addEventListener('MozFlyoutPanelShowing', this, false);
+    window.addEventListener('MozFlyoutPanelHiding', this, false);
 
 #if MOZ_UPDATE_CHANNEL != release
     let defaults = Services.prefs.getDefaultBranch("");
@@ -59,9 +60,13 @@ let AboutFlyoutPanel = {
     switch (aEvent.type) {
       case 'MozFlyoutPanelShowing':
 #ifdef MOZ_UPDATER
-        onUnload();
         this.appUpdater = new appUpdater();
         gAppUpdater = this.appUpdater;
+#endif
+        break;
+      case 'MozFlyoutPanelHiding':
+#ifdef MOZ_UPDATER
+        onUnload();
 #endif
         break;
     }
@@ -83,7 +88,7 @@ function onUnload(aEvent) {
   // Safe to call even when there isn't a download in progress.
   gAppUpdater.removeDownloadListener();
   gAppUpdater = null;
-  AboutFlyout.appUpdater = null;
+  AboutFlyoutPanel.appUpdater = null;
 }
 
 function appUpdater()