fixes #221, initial sync*, async, async* implementation

lib/runtime/dart/async.js

 dart_library.library('dart/async', null, /* Imports */[
   "dart_runtime/dart",
   'dart/core',
   'dart/_internal',
   'dart/collection'
 ], /* Lazy imports */[
   'dart/_isolate_helper'
 ], function(exports, dart, core, _internal, collection, _isolate_helper) {
   'use strict';
   let dartx = dart.dartx;
@@ skipping 4052 matching lines @@
         }
       }
       [_clear]() {
         this[_subscription] = null;
         this[_futureOrPrefetch] = null;
         this[_current] = null;
         this[_state] = _StreamIteratorImpl$()._STATE_DONE;
       }
       cancel() {
         let subscription = this[_subscription];
+        if (subscription == null)
+          return null;
         if (this[_state] == _StreamIteratorImpl$()._STATE_MOVING) {
           let hasNext = dart.as(this[_futureOrPrefetch], _Future$(core.bool));
           this[_clear]();
           hasNext[_complete](false);
         } else {
           this[_clear]();
         }
         return subscription.cancel();
       }
       [_onData](data) {
@@ skipping 1828 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;
 });