Fix racey crash in TimelineDurationScope

runtime/vm/timeline.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 <cstdlib>
 
 #include "vm/atomic.h"
 #include "vm/isolate.h"
 #include "vm/json_stream.h"
 #include "vm/lockers.h"
@@ skipping 493 matching lines @@
   Init();
 }
 
 
 TimelineDurationScope::~TimelineDurationScope() {
   if (!enabled_) {
     FreeArguments();
     return;
   }
   TimelineEvent* event = stream_->StartEvent();
+  if (event == NULL) {
+    // Stream is now disabled.
+    FreeArguments();
+    return;
+  }
   ASSERT(event != NULL);
   event->Duration(label_, timestamp_, OS::GetCurrentTraceMicros());
   event->StealArguments(arguments_length_, arguments_);
   event->Complete();
   arguments_length_ = 0;
   arguments_ = NULL;
 }
 
 
 void TimelineDurationScope::Init() {
@@ skipping 938 matching lines @@
       // If an isolate was specified, skip events from other isolates.
       continue;
     }
     ASSERT(event->event_as_json() != NULL);
     result = zone->ConcatStrings(result, event->event_as_json());
   }
   return result;
 }
 
 }  // namespace dart