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

runtime/lib/event_loop_patch.dart

Index: runtime/lib/event_loop_patch.dart
diff --git a/runtime/lib/event_loop_patch.dart b/runtime/lib/event_loop_patch.dart
deleted file mode 100644
index fe04c060bed955d4c596ac7d6bd1a4ffd30eebe6..0000000000000000000000000000000000000000
--- a/runtime/lib/event_loop_patch.dart
+++ /dev/null
@@ -1,10 +0,0 @@
-// 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 _enqueueImmediate(void callback()) {
-    // TODO(9001): don't use the Timer to enqueue the immediate callback.
-    _createTimer(Duration.ZERO, callback);
-  }
-}