Use a _FutureListener separate from the _Future to hold listeners.

sdk/lib/async/schedule_microtask.dart

Index: sdk/lib/async/schedule_microtask.dart
diff --git a/sdk/lib/async/schedule_microtask.dart b/sdk/lib/async/schedule_microtask.dart
index 7e1c82d9b382cc0631cae41b07e45ae7b34c94ba..cc0931d40cc5e9ee02f5b9a93dc641bd851b9c1e 100644
--- a/sdk/lib/async/schedule_microtask.dart
+++ b/sdk/lib/async/schedule_microtask.dart
@@ -83,10 +83,8 @@ void _scheduleAsyncCallback(callback) {
 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;