Fix flaky message queue test.
Fix flaky message queue test.

The test assumes that MessageQueue::Dequeue(0) cannot return NULL.
However, MessageQueue::Dequeue(0) returns NULL in case of spurious
wakeup.

R=turnidge@google.com,sgjesse@google.com
BUG=dartbug.com/2004
TEST=

Review URL: https://chromiumcodereview.appspot.com//10052011

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@6453 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk