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

tests/lib/async/zone_run_guarded_test.dart

Index: tests/lib/async/zone_run_guarded_test.dart
diff --git a/tests/lib/async/zone_run_guarded_test.dart b/tests/lib/async/zone_run_guarded_test.dart
index 60ffe5a3eccf11b6b1c648607855ea70407dfb81..4145443a947652f51a963a23e9c31e3868fb6acf 100644
--- a/tests/lib/async/zone_run_guarded_test.dart
+++ b/tests/lib/async/zone_run_guarded_test.dart
@@ -57,7 +57,7 @@ main() {
   result = forked.runGuarded(() {
     Expect.identical(forked, Zone.current);
     events.add("run closure");
-    scheduleMicrotask(() {
+    forked.scheduleMicrotask(() {
       events.add("run closure 2");
       Expect.identical(forked, Zone.current);
       done.complete(true);
@@ -76,4 +76,4 @@ main() {
         events);
     asyncEnd();
   });
-}
+}