Unify dart:isolate.

lib/isolate/timer_hook.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.
-
-typedef Timer _TimerFactoryClosure(int milliSeconds,
-                                   void callback(Timer timer),
-                                   bool repeating);
-
-// _TimerFactory provides a hook which allows various implementations of this
-// library to provide a concrete class for the Timer interface.
-class _TimerFactory {
-
-  factory Timer(int milliSeconds, void callback(Timer timer)) {
-    if (_factory == null) {
-      throw new UnsupportedOperationException("Timer interface not supported.");
-    }
-    return _factory(milliSeconds, callback, false);
-  }
-
-  factory Timer.repeating(int milliSeconds, void callback(Timer timer)) {
-    if (_factory == null) {
-      throw new UnsupportedOperationException("Timer interface not supported.");
-    }
-    return _factory(milliSeconds, callback, true);
-  }
-
-  static _TimerFactoryClosure _factory;
-}
-
-void _setTimerFactoryClosure(_TimerFactoryClosure closure) {
-  _TimerFactory._factory = closure;
-}