fixes #206, add checking for unary ops

lib/runtime/dart/async.js

 var async = dart.defineLibrary(async, {});
 var core = dart.import(core);
 var _internal = dart.import(_internal);
 var _js_helper = dart.lazyImport(_js_helper);
 var _isolate_helper = dart.lazyImport(_isolate_helper);
 var collection = dart.import(collection);
 (function(exports, core, _internal, _js_helper, _isolate_helper, collection) {
   'use strict';
   function _invokeErrorHandler(errorHandler, error, stackTrace) {
     if (dart.is(errorHandler, ZoneBinaryCallback)) {
@@ skipping 1378 matching lines @@
         if (core.identical(this[_next], this[_previous])) {
           _runGuarded(this[_onListen]);
         }
         return dart.as(subscription, StreamSubscription$(T));
       }
       [_recordCancel](subscription) {
         dart.as(subscription, StreamSubscription$(T));
         if (core.identical(subscription[_next], subscription))
           return null;
         dart.assert(!dart.notNull(core.identical(subscription[_next], subscription)));
-        if (subscription[_isFiring]) {
+        if (dart.as(subscription[_isFiring], core.bool)) {
           dart.dcall(subscription[_setRemoveAfterFiring]);
         } else {
           dart.assert(!dart.notNull(core.identical(subscription[_next], subscription)));
           this[_removeListener](dart.as(subscription, _BroadcastSubscription$(T)));
           if (!dart.notNull(this[_isFiring]) && dart.notNull(this[_isEmpty])) {
             this[_callOnCancel]();
           }
         }
         return null;
       }
@@ skipping 4491 matching lines @@
   exports.ScheduleMicrotaskHandler = ScheduleMicrotaskHandler;
   exports.CreateTimerHandler = CreateTimerHandler;
   exports.CreatePeriodicTimerHandler = CreatePeriodicTimerHandler;
   exports.PrintHandler = PrintHandler;
   exports.ForkHandler = ForkHandler;
   exports.ZoneSpecification = ZoneSpecification;
   exports.ZoneDelegate = ZoneDelegate;
   exports.Zone = Zone;
   exports.runZoned = runZoned;
 })(async, core, _internal, _js_helper, _isolate_helper, collection);