Expose Zones.

sdk/lib/async/event_loop.dart

Index: sdk/lib/async/event_loop.dart
diff --git a/sdk/lib/async/event_loop.dart b/sdk/lib/async/event_loop.dart
index 9462460ecee20605358ec6f0bd7d8e6ae8abd039..94137da655b61c61739078757e06e6c8a36c0a9d 100644
--- a/sdk/lib/async/event_loop.dart
+++ b/sdk/lib/async/event_loop.dart
@@ -55,8 +55,14 @@ void _scheduleAsyncCallback(callback) {
  *     }
  */
 void runAsync(void callback()) {
-  _Zone currentZone = _Zone._current;
-  currentZone.runAsync(callback, currentZone);
+  if (Zone.current == Zone.ROOT) {
+    // No need to bind the callback. We know that the root's runAsync will
+    // be invoked in the root zone.
+    Zone.current.scheduleMicrotask(callback);
+    return;
+  }
+  Zone.current.scheduleMicrotask(
+      Zone.current.bindCallback(callback, runGuarded: true));
 }
 
 class _AsyncRun {