Fix some service protocol isolate lifecycle races

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

Index: runtime/observatory/lib/src/app/application.dart
diff --git a/runtime/observatory/lib/src/app/application.dart b/runtime/observatory/lib/src/app/application.dart
index 644b65912cb20d2426e75cf0062dfdc76fdc44b0..e2d51263849743ff777676ebe6076a1557791c49 100644
--- a/runtime/observatory/lib/src/app/application.dart
+++ b/runtime/observatory/lib/src/app/application.dart
@@ -87,6 +87,8 @@ class ObservatoryApplication extends Observable {
   }
 
   void _onEvent(ServiceEvent event) {
+    assert(event.kind != ServiceEvent.kNone);
+
     switch(event.kind) {
       case ServiceEvent.kVMUpdate:
       case ServiceEvent.kIsolateStart: