don't run dart gen on JS patch code

test/dart_codegen/expect/async/schedule_microtask.dart

 part of dart.async;
 
 typedef void _AsyncCallback();
 class _AsyncCallbackEntry {
   final _AsyncCallback callback;
   _AsyncCallbackEntry next;
   _AsyncCallbackEntry(this.callback);
 }
 _AsyncCallbackEntry _nextCallback;
 _AsyncCallbackEntry _lastCallback;
@@ skipping 52 matching lines @@
     _rootScheduleMicrotask(null, null, DDC$RT.cast(_ROOT_ZONE, dynamic, Zone,
         "CastGeneral",
         """line 130, column 40 of dart:async/schedule_microtask.dart: """,
         _ROOT_ZONE is Zone, true), callback);
     return;
   }
   Zone.current
       .scheduleMicrotask(Zone.current.bindCallback(callback, runGuarded: true));
 }
 class _AsyncRun {
-  static void _scheduleImmediate(void callback()) {
-    scheduleImmediateClosure(callback);
-  }
+  external static void _scheduleImmediate(void callback());
 }