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

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

Index: runtime/bin/vmservice/observatory/lib/src/app/application.dart
diff --git a/runtime/bin/vmservice/observatory/lib/src/app/application.dart b/runtime/bin/vmservice/observatory/lib/src/app/application.dart
index c3018c931d3ccb909f9009eaf5e3bd198198649b..dec1e28fca47efc1dd91ec616b6151605450241b 100644
--- a/runtime/bin/vmservice/observatory/lib/src/app/application.dart
+++ b/runtime/bin/vmservice/observatory/lib/src/app/application.dart
@@ -87,6 +87,10 @@ class ObservatoryApplication extends Observable {
         notifications.add(event);
         break;
 
+      case 'GC':
+        // Ignore GC events for now.
+        break;
+
       default:
         // Ignore unrecognized events.
         Logger.root.severe('Unrecognized event: $event');