- Move the Timer interface to the dart:isolate library so that it

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;
+}