Handle spurious wakeups properly.

runtime/vm/message_queue.h

 // 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.
 
 #ifndef VM_MESSAGE_QUEUE_H_
 #define VM_MESSAGE_QUEUE_H_
 
 #include "include/dart_api.h"
 #include "vm/thread.h"
 
@@ skipping 30 matching lines @@
 
 
 // There is a message queue per isolate.
 class MessageQueue {
  public:
   MessageQueue() : head_(NULL), tail_(NULL) {}
   ~MessageQueue();
 
   void Enqueue(PortMessage* msg);
 
-  // May block if no message is available.
+  // Gets the next message from the message queue, possibly blocking
+  // if no message is available. 'millis' is a timeout in
+  // milliseconds. If 'millis' is 0, then this means to block
+  // indefinitely. May block if no message is available. May return
+  // NULL even if 'millis' is 0 due to spurious wakeups.
   PortMessage* Dequeue(int64_t millis);
 
   void Flush(Dart_Port port);
   void FlushAll();
 
  private:
   friend class MessageQueueTestPeer;
 
   Monitor monitor_;
   PortMessage* head_;
   PortMessage* tail_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageQueue);
 };
 
 }  // namespace dart
 
 #endif  // VM_MESSAGE_QUEUE_H_