Add a new Sentinel type for sentinel responses. (They are not Null).

runtime/bin/vmservice/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;
 
 /// The observatory application. Instances of this are created and owned
 /// by the observatory_application custom element.
 class ObservatoryApplication extends Observable {
   static ObservatoryApplication app;
@@ skipping 69 matching lines @@
         event.isolate.reloadBreakpoints();
         break;
 
       case 'BreakpointReached':
       case 'IsolateInterrupted':
       case 'ExceptionThrown':
         removePauseEvents(event.isolate);
         notifications.add(event);
         break;
 
+      case 'GC':
+        // Ignore GC events for now.
+        break;
+
       default:
         // Ignore unrecognized events.
         Logger.root.severe('Unrecognized event: $event');
         break;
     }
   }
 
   void _registerPages() {
     // Register ClassTreePage.
     _pageRegistry.add(new ClassTreePage(this));
@@ skipping 104 matching lines @@
 
   _vmDisconnected(VM vm) {
     if (this.vm != vm) {
       // This disconnect event occured *after* a new VM was installed.
       return;
     }
     this.vm = null;
     locationManager.go(locationManager.makeLink('/vm-connect/'));
   }
 }