Fixed typo in comment

runtime/observatory/lib/src/app/application.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of app;
 
 class Notification {
   Notification.fromEvent(this.event);
   Notification.fromException(this.exception, this.stacktrace);
 
@@ skipping 167 matching lines @@
         return;
       }
     }
     throw new FallThroughError();
   }
 
   /// Set the Observatory application page.
   void _installPage(Page page) {
     assert(page != null);
     if (currentPage == page) {
-      // Already isntalled.
+      // Already installed.
       return;
     }
     if (currentPage != null) {
       Logger.root.info('Uninstalling page: $currentPage');
       currentPage.onUninstall();
       // Clear children.
       rootElement.children.clear();
     }
     Logger.root.info('Installing page: $page');
     try {
@@ skipping 42 matching lines @@
     }
 
     // TODO(turnidge): Report this failure via analytics.
     Logger.root.warning('Caught exception: ${e}\n${st}');
     notifications.add(new Notification.fromException(e, st));
   }
 
   // This map keeps track of which curly-blocks have been expanded by the user.
   Map<String,bool> expansions = {};
 }