Add a test for timers in child isolates.

tools/dom/src/Timer.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of html;
 
 // TODO(antonm): support not DOM isolates too.
 class _Timer implements Timer {
   final canceller;
 
@@ skipping 15 matching lines @@
   Timer timer;
   final int id = maker(() { callback(timer); }, milliSeconds);
   timer = new _Timer(() { canceller(id); });
   return timer;
 };
 
 class _PureIsolateTimer implements Timer {
   final ReceivePort _port = new ReceivePort();
   SendPort _sendPort; // Effectively final.
 
+  static SendPort _SEND_PORT;
+
   _PureIsolateTimer(int milliSeconds, callback, repeating) {
     _sendPort = _port.toSendPort();
     _port.receive((msg, replyTo) {
       assert(msg == _TIMER_PING);
       callback(this);
       if (!repeating) _cancel();
     });
-    _HELPER_ISOLATE_PORT.then((port) {
-      port.send([_NEW_TIMER, milliSeconds, repeating], _sendPort);
-    });
+
+    _send([_NEW_TIMER, milliSeconds, repeating]);
   }
 
   void cancel() {
     _cancel();
-    _HELPER_ISOLATE_PORT.then((port) {
-      port.send([_CANCEL_TIMER], _sendPort);
-    });
+    _send([_CANCEL_TIMER]);
   }
 
   void _cancel() {
     _port.close();
   }
+
+  // Tricky part.
+  // Once _HELPER_ISOLATE_PORT gets resolved, it will still delay in .then
+  // and to delay Timer.run is used. However, Timer.run will try to register
+  // another Timer and here we got stuck: event cannot be posted as then
+  // callback is not executed because it's delayed with timer.
+  // Therefore once future is resolved, it's unsafe to call .then on it
+  // in Timer code.

[vsm, 2013/04/01 15:27:32]

Thanks, this looks good.

+  _send(msg) {
+    if (_SEND_PORT != null) {
+      _SEND_PORT.send(msg, _sendPort);
+    } else {
+      _HELPER_ISOLATE_PORT.then((port) {
+        _SEND_PORT = port;
+        _SEND_PORT.send(msg, _sendPort);
+      });
+    }
+  }
 }
 
 get _pureIsolateTimerFactoryClosure =>
     ((int milliSeconds, void callback(Timer time), bool repeating) =>
         new _PureIsolateTimer(milliSeconds, callback, repeating));