Make the repeating argument to timers optional.

runtime/bin/timer_impl.dart

Index: runtime/bin/timer_impl.dart
diff --git a/runtime/bin/timer_impl.dart b/runtime/bin/timer_impl.dart
index 78baa8c3fbe812d1944ba4070ce844cae6150ea4..52dc7cc7d09dd83f5686ac9171fbd48e57c3362d 100644
--- a/runtime/bin/timer_impl.dart
+++ b/runtime/bin/timer_impl.dart
@@ -14,9 +14,9 @@ class _Timer implements Timer {
    */
   static final int _NO_TIMER = -1;
 
-  factory _Timer(void callback(Timer timer),
-                 int milliSeconds,
-                 bool repeating) {
+  static Timer _createTimer(void callback(Timer timer),
+                           int milliSeconds,
+                           bool repeating) {
     EventHandler._start();
     if (_timers === null) {
       _timers = new DoubleLinkedQueue<_Timer>();
@@ -31,6 +31,14 @@ class _Timer implements Timer {
     return timer;
   }
 
+  factory _Timer(void callback(Timer timer), int milliSeconds) {
+    return _createTimer(callback, milliSeconds, false);
+  }
+
+  factory _Timer.repeating(void callback(Timer timer), int milliSeconds) {
+    return _createTimer(callback, milliSeconds, true);
+  }
+
   _Timer._internal() {}
 
   void _clear() {