Quick fix.

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 34 matching lines @@
       callback(this);
       if (!repeating) _cancel();
     });
     _HELPER_ISOLATE_PORT.then((port) {
       port.send([_NEW_TIMER, milliSeconds, repeating], _sendPort);
     });
   }
 
   void cancel() {
     _cancel();
-    _HELPER_ISOLATE_PORT.send([_CANCEL_TIMER], _sendPort);
+    _HELPER_ISOLATE_PORT.then((port) {
+      port.send([_CANCEL_TIMER], _sendPort);
+    });
   }
 
   void _cancel() {
     _port.close();
   }
 
   static final Future<SendPort> _HELPER_ISOLATE_PORT =
       spawnDomFunction(_helperIsolateMain);
 }
 
@@ skipping 11 matching lines @@
           new Timer(duration, callback);
     } else if (cmd == _CANCEL_TIMER) {
       _TIMER_REGISTRY.remove(replyTo).cancel();
     }
   });
 }
 
 get _pureIsolateTimerFactoryClosure =>
     ((int milliSeconds, void callback(Timer time), bool repeating) =>
         new _PureIsolateTimer(milliSeconds, callback, repeating));