Use a _FutureListener separate from the _Future to hold listeners.

sdk/lib/async/schedule_microtask.dart

 // Copyright (c) 2013, 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.
 
 part of dart.async;
 
 typedef void _AsyncCallback();
 
 class _AsyncCallbackEntry {
   final _AsyncCallback callback;
@@ skipping 65 matching lines @@
 
 /**
  * Schedules a callback to be called before all other currently scheduled ones.
  *
  * This callback takes priority over existing scheduled callbacks.
  * It is only used internally to give higher priority to error reporting.
  */
 void _schedulePriorityAsyncCallback(callback) {
   _AsyncCallbackEntry entry = new _AsyncCallbackEntry(callback);
   if (_nextCallback == null) {
-    _nextCallback = _lastCallback = _lastPriorityCallback = entry;
-    if (!_isInCallbackLoop) {
-      _AsyncRun._scheduleImmediate(_asyncRunCallback);
-    }
+    _scheduleAsyncCallback(callback);
+    _lastPriorityCallback = _lastCallback;
   } else if (_lastPriorityCallback == null) {
     entry.next = _nextCallback;
     _nextCallback = _lastPriorityCallback = entry;
   } else {
     entry.next = _lastPriorityCallback.next;
     _lastPriorityCallback.next = entry;
     _lastPriorityCallback = entry;
     if (entry.next == null) {
       _lastCallback = entry;
     }
@@ skipping 33 matching lines @@
     return;
   }
   Zone.current.scheduleMicrotask(
       Zone.current.bindCallback(callback, runGuarded: true));
 }
 
 class _AsyncRun {
   /** Schedule the given callback before any other event in the event-loop. */
   external static void _scheduleImmediate(void callback());
 }