Fix more races in MessageQueue_WaitNotify. My notifies could show up

Fix more races in MessageQueue_WaitNotify.  My notifies could show up
before my waits, and I would timeout.
Committed: https://code.google.com/p/dart/source/detail?r=536

[turnidge, 2011-10-18 21:41:18]

TBR=iposva

[siva, 2011-10-18 22:19:59]

DBC.

http://codereview.chromium.org/8347013/diff/1/runtime/vm/message_queue_test.cc
File runtime/vm/message_queue_test.cc (right):

http://codereview.chromium.org/8347013/diff/1/runtime/vm/message_queue_test.c...
runtime/vm/message_queue_test.cc:73: }
The update to shared_queue could be under a lock:
{
  MonitorLocker ml(sync);
  shared_queue = queue;
  ml.notify();
}

http://codereview.chromium.org/8347013/diff/1/runtime/vm/message_queue_test.c...
runtime/vm/message_queue_test.cc:79: }
The check for peer.HasMessage() could be under a lock,
Also we should probably be using a different monitor for
this as the notify could get eaten by the same thread that
did the notify.

{
  MonitorLocker ml(sync1);
  while (!peer.HasMessage()) {
    ml.Wait();
  }
}

http://codereview.chromium.org/8347013/diff/1/runtime/vm/message_queue_test.c...
runtime/vm/message_queue_test.cc:113: }
The check for shared queue could be under a lock:
{
  MonitorLocker ml(sync);
  while (shared_queue == NULL) {
    ml.Wait();
  }
}