fix subscription issue

com.unity.uiwidgets/Runtime/async/stream_controller.cs

         public void _recordResume(StreamSubscription<T> subscription) {
         }
 
-        public Future _recordCancel(StreamSubscription<T> subscription) => null;
+        public virtual Future _recordCancel(StreamSubscription<T> subscription) => null;
 
         public abstract void _add(T data);
 
             return subscription;
         }
 
-        Future _recordCancel(StreamSubscription<T> subscription) {
+        public override Future _recordCancel(StreamSubscription<T> subscription) {
             // When we cancel, we first cancel any stream being added,
             // Then we call `onCancel`, and finally the _doneFuture is completed.
             // If either of addStream's cancel or `onCancel` returns a future,

com.unity.uiwidgets/Runtime/async/stream_impl.cs

             handleData = handleData ?? _stream._nullDataHandler;
             // TODO(floitsch): the return type should be 'void', and the type
             // should be inferred.
-            _onData = d => _zone.registerUnaryCallback(data => {
-                handleData((T) data);
-                return default;
-            });
+            _onData = d => {
+                _zone.registerUnaryCallback(data => 
+                {
+                    handleData((T) data);
+                    return default;
+                }).Invoke(d);
+            };
         }
 
         // Siyao: c# does not support convert action