Restore _TimerFactory class.

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)) {
-    if (_factory == null) {
+    if (_TimerFactory._factory == null) {
       throw new UnsupportedOperationException("Timer interface not supported.");
     }
-    return _factory(milliSeconds, callback, false);
+    return _TimerFactory._factory(milliSeconds, callback, false);
   }
 
   /**
    * Creates a new repeating timer. The [callback] is invoked every
    * [milliSeconds] millisecond until cancelled.
    */
   factory Timer.repeating(int milliSeconds, void callback(Timer timer)) {
-    if (_factory == null) {
+    if (_TimerFactory._factory == null) {
       throw new UnsupportedOperationException("Timer interface not supported.");
     }
-    return _factory(milliSeconds, callback, true);
+    return _TimerFactory._factory(milliSeconds, callback, true);
   }
 
   /**
    * Cancels the timer.
    */
   void cancel();
-
-  static _TimerFactoryClosure _factory;
 }
 
 // TODO(ajohnsen): Patch timer once we have support for patching named
 //                 factory constructors in the VM.
 
 typedef Timer _TimerFactoryClosure(int milliSeconds,
                                    void callback(Timer timer),
                                    bool repeating);
 
+class _TimerFactory {
+  static _TimerFactoryClosure _factory;
+}
+
 void _setTimerFactoryClosure(_TimerFactoryClosure closure) {
-  Timer._factory = closure;
+  _TimerFactory._factory = closure;
 }