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

lib/runtime/dart/async.js

Index: lib/runtime/dart/async.js
diff --git a/lib/runtime/dart/async.js b/lib/runtime/dart/async.js
index 99506c10940f291f7af116a84b26f3d94c781155..3e3fc75938b0e5466540397db5011e9afe2d1b45 100644
--- a/lib/runtime/dart/async.js
+++ b/lib/runtime/dart/async.js
@@ -4070,6 +4070,8 @@ dart_library.library('dart/async', null, /* Imports */[
       }
       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]();