Use while(condition) around monitor wait, in file_system_watcher_macos.

This fixes a potential threading-issue, where the wacther could be used before it was fully initialized, if 'wait' woke up before notify.

BUG=
R=sgjesse@google.com

Review URL: https://codereview.chromium.org//39713002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@29163 260f80e4-7a28-3924-810f-c04153c831b5

runtime/bin/file_system_watcher_macos.cc

@@ -136,7 +136,9 @@ class FSEventsWatcher {
     if (watcher == NULL) {
       watcher_monitor->Enter();
       watcher = new FSEventsWatcher();
-      watcher_monitor->Wait(Monitor::kNoTimeout);
+      while (watcher->run_loop_ == NULL) {
+        watcher_monitor->Wait(Monitor::kNoTimeout);
+      }
       watcher_monitor->Exit();
     }
     watcher->users_++;