Fix some potential data races based on output from tsan.

runtime/vm/thread_interrupter.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 "vm/thread_interrupter.h"
 
 #include "vm/flags.h"
 #include "vm/lockers.h"
 #include "vm/os.h"
 #include "vm/simulator.h"
@@ skipping 145 matching lines @@
     // Signal to main thread we are ready.
     MonitorLocker startup_ml(monitor_);
     OSThread* os_thread = OSThread::Current();
     ASSERT(os_thread != NULL);
     interrupter_thread_id_ = os_thread->join_id();
     thread_running_ = true;
     startup_ml.Notify();
   }
   {
     intptr_t interrupted_thread_count = 0;
+    MonitorLocker wait_ml(monitor_);
     current_wait_time_ = interrupt_period_;
-    MonitorLocker wait_ml(monitor_);
     while (!shutdown_) {
       intptr_t r = wait_ml.WaitMicros(current_wait_time_);
 
       if (shutdown_) {
         break;
       }
 
       if ((r == Monitor::kNotified) && InDeepSleep()) {
         // Woken up from deep sleep.
         ASSERT(interrupted_thread_count == 0);
@@ skipping 44 matching lines @@
     // Signal to main thread we are exiting.
     MonitorLocker shutdown_ml(monitor_);
     thread_running_ = false;
     shutdown_ml.Notify();
   }
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart