Add isActive field on Timer.

sdk/lib/async/timer.dart

Index: sdk/lib/async/timer.dart
diff --git a/sdk/lib/async/timer.dart b/sdk/lib/async/timer.dart
index 3544728ece9ad5f242f97da7d80a2104ca8e89a9..0ea89fb4ca4c83027f60ad97e27e030731fcc050 100644
--- a/sdk/lib/async/timer.dart
+++ b/sdk/lib/async/timer.dart
@@ -57,6 +57,11 @@ abstract class Timer {
    * Cancels the timer.
    */
   void cancel();
+
+  /**
+   * Returns true if the timer is not done, that is it will run the callback.

[floitsch, 2013/07/01 12:49:43]

Returns whether the timer is still active.

A non-periodic callback is active if the callback has not been executed, and the
timer has not been canceled.

A periodic callback is active if it has not been canceled.

[zarah, 2013/07/01 16:15:35]

Done.

+   */
+  bool get isActive;
 }
 
 external Timer _createTimer(Duration duration, void callback());