Fixed a problem with spurious wakeups.

runtime/vm/thread_test.cc

 // Copyright (c) 2011, 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/assert.h"
 #include "vm/isolate.h"
 #include "vm/unit_test.h"
 #include "vm/thread.h"
 
 namespace dart {
@@ skipping 27 matching lines @@
   Monitor* monitor = new Monitor();
   monitor->Enter();
   monitor->Exit();
 
   {
     MonitorLocker ml(monitor);
     int64_t start = OS::GetCurrentTimeMillis();
     int64_t wait_time = 2017;
     Monitor::WaitResult wait_result = ml.Wait(wait_time);
     int64_t stop = OS::GetCurrentTimeMillis();
+    // Note: This may fail due to spurious wakeups in pthread_cond_wait().
     EXPECT_EQ(Monitor::kTimedOut, wait_result);
     const int kAcceptableTimeJitter = 20;  // Measured in milliseconds.
     EXPECT_LE(wait_time - kAcceptableTimeJitter, stop - start);
     const int kAcceptableWakeupDelay = 120;  // Measured in milliseconds.
     EXPECT_GE(wait_time + kAcceptableWakeupDelay, stop - start);
   }
   // The isolate shutdown and the destruction of the mutex are out-of-order on
   // purpose.
   isolate->Shutdown();
   delete isolate;
   delete monitor;
 }
 
 }  // namespace dart