Rewrite Zone implementation to avoid recursively looking up stuff in the parent chain.

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 41 matching lines @@
  * Runs a function 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() {
+ *       Timer.run(() { print("executed"); });  // Will never be executed.
+ *       foo() {
+ *         scheduleMicrotask(foo);  // Schedules [foo] in front of other events.
+ *       }
  *       foo();
  *     }
  *
  * ## Other resources
  *
  * * [The Event Loop and Dart](https://www.dartlang.org/articles/event-loop/):
  * Learn how Dart handles the event queue and microtask queue, so you can write
  * better asynchronous code with fewer surprises.
  */
 void scheduleMicrotask(void callback()) {
-  if (Zone.current == Zone.ROOT) {
+  if (identical(_ROOT_ZONE, Zone.current)) {
     // No need to bind the callback. We know that the root's scheduleMicrotask
     // will be invoked in the root zone.
-    Zone.current.scheduleMicrotask(callback);
+    _rootScheduleMicrotask(null, null, _ROOT_ZONE, callback);
     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());
 }