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

runtime/lib/schedule_microtask_patch.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.
+
+patch class _AsyncRun {
+  /* patch */ static void _scheduleImmediate(void callback()) {
+    if (_ScheduleImmediate._closure == null) {
+      // TODO(9001): don't default to using the Timer to enqueue the immediate
+      //             callback.
+      _createTimer(Duration.ZERO, callback);
+      return;
+    }
+    _ScheduleImmediate._closure(callback);
+  }
+}
+
+typedef void _ScheduleImmediateClosure(void callback());
+
+class _ScheduleImmediate {
+  static _ScheduleImmediateClosure _closure;
+}
+
+void _setScheduleImmediateClosure(_ScheduleImmediateClosure closure) {
+  _ScheduleImmediate._closure = closure;
+}