Make the repeating argument to timers optional.

runtime/bin/timer.dart

 // Copyright (c) 2011, 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.
 
 interface Timer factory _Timer{
 
   /*
    * Creates a new timer. The [callback] callback is invoked after
    * [milliSeconds] milliseconds. If [repeating] is set, the timer
    * is repeated every [milliSeconds] milliseconds until cancelled.
+   * [repeating] is false by default.
    */
-  Timer(void callback(Timer timer), int milliSeconds, bool repeating);
+  Timer(void callback(Timer timer), int milliSeconds, [bool repeating]);
 
   /*
    * Cancels the timer.
    */
   void cancel();
 }