Make the root zone's schduleMicrotask and create[Periodic]Timer bind the callback.

tests/lib/async/zone_empty_description2_test.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.
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'dart:async';
 
 testEmptyZoneSpecification() {
   Expect.identical(Zone.ROOT, Zone.current);
   Zone forked = Zone.current.fork();
   Expect.isFalse(identical(Zone.ROOT, forked));
 
   asyncStart();
   bool timerDidRun = false;
   forked.createTimer(const Duration(milliseconds: 20), () {
-    // The createTimer functions on the Zone binds the closures.
+    // The createTimer function on the Zone binds the closures.
     Expect.identical(forked, Zone.current);
     timerDidRun = true;
     asyncEnd();
   });
   Expect.identical(Zone.ROOT, Zone.current);
 
   asyncStart();
   int periodicTimerCount = 0;
   forked.createPeriodicTimer(const Duration(milliseconds: 20), (Timer timer) {
     periodicTimerCount++;
     if (periodicTimerCount == 4) {
       timer.cancel();
       asyncEnd();
     }
-    // The createPeriodicTimer functions on the Zone binds the closures.
+    // The createPeriodicTimer function on the Zone binds the closures.
     Expect.identical(forked, Zone.current);
   });
   Expect.identical(Zone.ROOT, Zone.current);
 }
 
 main() {
   testEmptyZoneSpecification();
 }