Enable inference in the core libs

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;
  _AsyncCallbackEntry _lastPriorityCallback;
  bool _isInCallbackLoop = false;
@@ skipping 44 matching lines @@
   entry.next = _lastPriorityCallback.next;
    _lastPriorityCallback.next = entry;
    _lastPriorityCallback = entry;
    if (entry.next == null) {
     _lastCallback = entry;
     }
   }
 }
  void scheduleMicrotask(void callback()) {
 if (identical(_ROOT_ZONE, Zone.current)) {
-  _rootScheduleMicrotask(null, null, DEVC$RT.cast(_ROOT_ZONE, dynamic, Zone, "CastGeneral", """line 130, column 40 of dart:async/schedule_microtask.dart: """, _ROOT_ZONE is Zone, true), callback);
+  _rootScheduleMicrotask(null, null, _ROOT_ZONE, callback);
    return;}
  Zone.current.scheduleMicrotask(Zone.current.bindCallback(callback, runGuarded: true));
 }
  class _AsyncRun {external static void _scheduleImmediate(void callback());
 }
  typedef void __t36();