Add isActive field on Timer.

tools/dom/src/native_DOMImplementation.dart

Index: tools/dom/src/native_DOMImplementation.dart
diff --git a/tools/dom/src/native_DOMImplementation.dart b/tools/dom/src/native_DOMImplementation.dart
index 89f139410824c907f28b772ff438896480913901..c3d9ad88d99ef728461332e6c378194c993dce3a 100644
--- a/tools/dom/src/native_DOMImplementation.dart
+++ b/tools/dom/src/native_DOMImplementation.dart
@@ -200,10 +200,16 @@ final _forwardingPrintClosure = _Utils.forwardingPrint;
 
 class _Timer implements Timer {
   final canceller;
+  bool _isDone = false;
 
   _Timer(this.canceller);
 
-  void cancel() { canceller(); }
+  void cancel() {
+    _isDone = true;
+    canceller();
+  }
+
+  bool get isActive => !_isDone;
 }
 
 get _timerFactoryClosure => (int milliSeconds, void callback(Timer timer), bool repeating) {
@@ -223,6 +229,7 @@ get _timerFactoryClosure => (int milliSeconds, void callback(Timer timer), bool
 };
 
 class _PureIsolateTimer implements Timer {
+  bool _isDone = false;
   final ReceivePort _port = new ReceivePort();
   SendPort _sendPort; // Effectively final.
 
@@ -232,6 +239,7 @@ class _PureIsolateTimer implements Timer {
     _sendPort = _port.toSendPort();
     _port.receive((msg, replyTo) {
       assert(msg == _TIMER_PING);
+      _isDone = !repeating;
       callback(this);
       if (!repeating) _cancel();
     });
@@ -245,12 +253,15 @@ class _PureIsolateTimer implements Timer {
   }
 
   void _cancel() {
+    _isDone = true;
     _port.close();
   }
 
   _send(msg) {
     _sendToHelperIsolate(msg, _sendPort);
   }
+
+  bool get isActive => !_isDone;
 }
 
 get _pureIsolateTimerFactoryClosure =>