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..ff7b07ed5bdb8a2568038769c2b5d064b1f66c24 100644
--- a/sdk/lib/async/timer.dart
+++ b/sdk/lib/async/timer.dart
@@ -57,6 +57,16 @@ abstract class Timer {
    * Cancels the timer.
    */
   void cancel();
+
+  /**
+   * Returns whether the timer is still active.
+   *
+   * A non-periodic timer is active if the callback has not been executed,
+   * and the timer has not been canceled.
+   *
+   * A periodic timer is active if it has not been canceled.
+   */
+  bool get isActive;
 }
 
 external Timer _createTimer(Duration duration, void callback());