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

tests/lib/async/zone_run_test.dart

Index: tests/lib/async/zone_run_test.dart
diff --git a/tests/lib/async/zone_run_test.dart b/tests/lib/async/zone_run_test.dart
index 2af32dfe9226d86cfb93faa906334c3ad49867d2..3c13c469f082de94000e2b5b1fc4e82eb922232a 100644
--- a/tests/lib/async/zone_run_test.dart
+++ b/tests/lib/async/zone_run_test.dart
@@ -44,7 +44,7 @@ main() {
   result = forked.run(() {
     Expect.identical(forked, Zone.current);
     events.add("run closure 2");
-    scheduleMicrotask(() {
+    forked.scheduleMicrotask(() {
       events.add("run closure 3");
       Expect.identical(forked, Zone.current);
       done.complete(true);
@@ -62,4 +62,4 @@ main() {
         events);
     asyncEnd();
   });
-}
+}