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..1d6ae20e8bf655d2f9c9e394a0d129f6c64559a8 100644
--- a/tools/dom/src/native_DOMImplementation.dart
+++ b/tools/dom/src/native_DOMImplementation.dart
@@ -200,29 +200,43 @@ final _forwardingPrintClosure = _Utils.forwardingPrint;
 
 class _Timer implements Timer {
   final canceller;
+  bool _isDone = false;
 
   _Timer(this.canceller);
 
-  void cancel() { canceller(); }
-}
+  factory _Timer.timerFactoryClosure(int milliSeconds,

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

Make this the default constructor. If you use the `canceller` field as indicator
if the timer is active (see below) then you can make this a non-factory
constructor.

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

Done.

+                                     void callback(Timer timer),
+                                     bool repeating) {
+    var maker;
+    var canceller;
+    if (repeating) {
+      maker = window._setInterval;
+      canceller = window._clearInterval;
+    } else {
+      maker = window._setTimeout;
+      canceller = window._clearTimeout;
+    }
+    Timer timer;
+    final int id = maker(() { _isDone = true; callback(timer);}, milliSeconds);

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

Don't put more than one statement on the same line.

You could also use the _canceller field (which should be private and called
"canceler") as indication instead of having an extra boolean field.

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

Done.

+    timer = new _Timer(() { canceller(id); });

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

Nit: it is annoying to me that we wrap the canceler in two closures.
Make `id` non-final, move it above the if and create the full canceler directly:
canceler = () => window._clearInterval(id);

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

Done.

+    return timer;
+  }
 
-get _timerFactoryClosure => (int milliSeconds, void callback(Timer timer), bool repeating) {
-  var maker;
-  var canceller;
-  if (repeating) {
-    maker = window._setInterval;
-    canceller = window._clearInterval;
-  } else {
-    maker = window._setTimeout;
-    canceller = window._clearTimeout;
+  void cancel() {
+    _isDone = true;
+    canceller();
   }
-  Timer timer;
-  final int id = maker(() { callback(timer); }, milliSeconds);
-  timer = new _Timer(() { canceller(id); });
-  return timer;
+
+  bool get isActive => !_isDone;
+}
+
+get _timerFactoryClosure =>
+    (int milliSeconds, void callback(Timer timer), bool repeating) {
+  return _Timer.timerFactoryClosure(milliseconds, callback, repeating);
 };
 
 class _PureIsolateTimer implements Timer {
+  bool _isDone = false;
   final ReceivePort _port = new ReceivePort();
   SendPort _sendPort; // Effectively final.
 
@@ -232,6 +246,7 @@ class _PureIsolateTimer implements Timer {
     _sendPort = _port.toSendPort();
     _port.receive((msg, replyTo) {
       assert(msg == _TIMER_PING);
+      _isDone = !repeating;
       callback(this);
       if (!repeating) _cancel();
     });
@@ -245,12 +260,15 @@ class _PureIsolateTimer implements Timer {
   }
 
   void _cancel() {
+    _isDone = true;
     _port.close();
   }
 
   _send(msg) {
     _sendToHelperIsolate(msg, _sendPort);
   }
+
+  bool get isActive => !_isDone;
 }
 
 get _pureIsolateTimerFactoryClosure =>