Make the repeating argument to timers optional.

runtime/bin/timer.dart

Index: runtime/bin/timer.dart
diff --git a/runtime/bin/timer.dart b/runtime/bin/timer.dart
index 34ea2c038c7f5973b5cdc06aea92278543482663..51a7c327704a2057ac010880855c72398edbb58e 100644
--- a/runtime/bin/timer.dart
+++ b/runtime/bin/timer.dart
@@ -8,8 +8,9 @@ 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.