Add warning for future maintainers of Timer.

dart/lib/isolate/timer.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 abstract class Timer {
   /**
    * Creates a new timer. The [callback] callback is invoked after
    * [milliSeconds] milliseconds.
    */
   factory Timer(int milliSeconds, void callback(Timer timer)) {
@@ skipping 24 matching lines @@
 //                 factory constructors in the VM.
 
 typedef Timer _TimerFactoryClosure(int milliSeconds,
                                    void callback(Timer timer),
                                    bool repeating);
 
 class _TimerFactory {
   static _TimerFactoryClosure _factory;
 }
 
+// TODO(ahe): Warning: this is NOT called by Dartium. Instead, it sets
+// [_TimerFactory._factory] directly.
 void _setTimerFactoryClosure(_TimerFactoryClosure closure) {
   _TimerFactory._factory = closure;
 }