Fix some service protocol isolate lifecycle races

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 69 matching lines @@
   void removePauseEvents(Isolate isolate) {
     notifications.removeWhere((notification) {
         var event = notification.event;
         return (event != null &&
                 event.isolate == isolate &&
                 event.isPauseEvent);
       });
   }
 
   void _onEvent(ServiceEvent event) {
+    assert(event.kind != ServiceEvent.kNone);
+
     switch(event.kind) {
       case ServiceEvent.kVMUpdate:
       case ServiceEvent.kIsolateStart:
       case ServiceEvent.kIsolateRunnable:
       case ServiceEvent.kIsolateUpdate:
       case ServiceEvent.kBreakpointAdded:
       case ServiceEvent.kBreakpointResolved:
       case ServiceEvent.kBreakpointRemoved:
       case ServiceEvent.kDebuggerSettingsUpdate:
         // Ignore for now.
@@ skipping 134 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 = {};
 }