Add factory-method for scheduleMicrotask, as well as a (failing) test.

sdk/lib/async/event_loop.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();
-
-bool _callbacksAreEnqueued = false;
-Queue<_AsyncCallback> _asyncCallbacks = new Queue<_AsyncCallback>();
-
-void _asyncRunCallback() {
-  // As long as we are iterating over the registered callbacks we don't
-  // unset the [_callbacksAreEnqueued] boolean.
-  while (!_asyncCallbacks.isEmpty) {
-    Function callback = _asyncCallbacks.removeFirst();
-    try {
-      callback();
-    } catch (e) {
-      _AsyncRun._enqueueImmediate(_asyncRunCallback);
-      rethrow;
-    }
-  }
-  // Any new callback must register a callback function now.
-  _callbacksAreEnqueued = false;
-}
-
-void _scheduleAsyncCallback(callback) {
-  // Optimizing a group of Timer.run callbacks to be executed in the
-  // same Timer callback.
-  _asyncCallbacks.add(callback);
-  if (!_callbacksAreEnqueued) {
-    _AsyncRun._enqueueImmediate(_asyncRunCallback);
-    _callbacksAreEnqueued = true;
-  }
-}
-
-/**
- * Runs the given [callback] asynchronously.
- *
- * Callbacks registered through this function are always executed in order and
- * are guaranteed to run before other asynchronous events (like [Timer] events,
- * or DOM events).
- *
- * Warning: it is possible to starve the DOM by registering asynchronous
- * callbacks through this method. For example the following program runs
- * the callbacks without ever giving the Timer callback a chance to execute:
- *
- *     Timer.run(() { print("executed"); });  // Will never be executed;
- *     foo() {
- *       scheduleMicrotask(foo);  // Schedules [foo] in front of other events.
- *     }
- *     main() {
- *       foo();
- *     }
- */
-void scheduleMicrotask(void callback()) {
-  if (Zone.current == Zone.ROOT) {
-    // No need to bind the callback. We know that the root's scheduleMicrotask
-    // will be invoked in the root zone.
-    Zone.current.scheduleMicrotask(callback);
-    return;
-  }
-  Zone.current.scheduleMicrotask(
-      Zone.current.bindCallback(callback, runGuarded: true));
-}
-
-/**
- * *DEPRECATED*. Use [scheduleMicrotask] instead.
- */
-@deprecated
-void runAsync(void callback()) {
-  scheduleMicrotask(callback);
-}
-
-class _AsyncRun {
-  /** Enqueues the given callback before any other event in the event-loop. */
-  external static void _enqueueImmediate(void callback());
-}