Observatory fixes:

runtime/vm/service_event.cc

 // Copyright (c) 2015, 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.
 
 #include "vm/service_event.h"
 
 namespace dart {
 
 // Translate from the legacy DebugEvent to a ServiceEvent.
 static ServiceEvent::EventKind TranslateEventKind(
@@ skipping 148 matching lines @@
   }
   if (top_frame() != NULL) {
     JSONObject jsFrame(&jsobj, "topFrame");
     top_frame()->PrintToJSONObject(&jsFrame);
     intptr_t index = 0;  // Avoid ambiguity in call to AddProperty.
     jsFrame.AddProperty("index", index);
   }
   if (exception() != NULL) {
     jsobj.AddProperty("exception", *(exception()));
   }
-  if (async_continuation() != NULL) {
+  if (async_continuation() != NULL && !async_continuation()->IsNull()) {
     jsobj.AddProperty("_asyncContinuation", *(async_continuation()));
   }
   if (inspectee() != NULL) {
     jsobj.AddProperty("inspectee", *(inspectee()));
   }
   if (gc_stats() != NULL) {
     jsobj.AddProperty("reason", Heap::GCReasonToString(gc_stats()->reason_));
     isolate()->heap()->PrintToJSONObject(Heap::kNew, &jsobj);
     isolate()->heap()->PrintToJSONObject(Heap::kOld, &jsobj);
   }
@@ skipping 15 matching lines @@
 
 
 void ServiceEvent::PrintJSONHeader(JSONObject* jsobj) const {
   ASSERT(jsobj != NULL);
   jsobj->AddProperty("type", "Event");
   jsobj->AddProperty("kind", KindAsCString());
   jsobj->AddProperty("isolate", isolate());
 }
 
 }  // namespace dart