Fix race discovered by Valgrind

runtime/vm/profiler_macos.cc

 // 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.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_MACOS)
 
 #include "vm/isolate.h"
 #include "vm/json_stream.h"
 #include "vm/profiler.h"
@@ skipping 115 matching lines @@
 }
 
 
 void ProfilerManager::ThreadMain(uword parameters) {
   ASSERT(initialized_);
   ASSERT(FLAG_profile);
   SignalHandler::Install(ProfileSignalAction);
   if (FLAG_trace_profiled_isolates) {
     OS::Print("ProfilerManager MacOS ready.\n");
   }
-  thread_running_ = true;
   {
     // Signal to main thread we are ready.
     ScopedMonitor startup_lock(start_stop_monitor_);
+    thread_running_ = true;
     startup_lock.Notify();
   }
   ScopedMonitor lock(monitor_);
   while (!shutdown_) {
     int64_t current_time = OS::GetCurrentTimeMicros();
     int64_t next_sample = SampleAndRescheduleIsolates(current_time);
     lock.WaitMicros(next_sample);
   }
   if (FLAG_trace_profiled_isolates) {
     OS::Print("ProfilerManager MacOS exiting.\n");
   }
-  thread_running_ = false;
   {
     // Signal to main thread we are exiting.
     ScopedMonitor shutdown_lock(start_stop_monitor_);
+    thread_running_ = false;
     shutdown_lock.Notify();
   }
 }
 
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)