Add isActive field on Timer.

sdk/lib/io/timer_impl.dart

Index: sdk/lib/io/timer_impl.dart
diff --git a/sdk/lib/io/timer_impl.dart b/sdk/lib/io/timer_impl.dart
index 73c7cae1c12ccad966449f43d93515a138917c10..34439a91243e81452280e6cf76166b5c1cb57c29 100644
--- a/sdk/lib/io/timer_impl.dart
+++ b/sdk/lib/io/timer_impl.dart
@@ -50,6 +50,8 @@ class _Timer extends LinkedListEntry<_Timer> implements Timer {
 
   bool get _repeating => _milliSeconds >= 0;
 
+  bool get isActive => (_callback != null);
+
 
   // Cancels a set timer. The timer is removed from the timer list and if
   // the given timer is the earliest timer the native timer is reset.
@@ -143,7 +145,11 @@ class _Timer extends LinkedListEntry<_Timer> implements Timer {
           // one of the later timers which will set the callback to
           // null.
           if (timer._callback != null) {
-            timer._callback(timer);
+            var callback = timer._callback;
+            if (!timer._repeating) {
+              timer._callback = null;

[floitsch, 2013/07/01 17:50:56]

Add comment: // Mark timer as inactive.

[zarah, 2013/07/02 11:22:01]

Done.

+            }
+            callback(timer);
             // Re-insert repeating timer if not canceled.
             if (timer._repeating && timer._callback != null) {
               timer._advanceWakeupTime();