fix unity compile error

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

         public override StreamSubscription<T> _subscribe(
             Action<T> onData, Action<object, string> onError, Action onDone, bool cancelOnError) {
             if (isClosed) {
-                onDone ??= _stream._nullDoneHandler;
+                onDone = onDone ?? _stream._nullDoneHandler;
                 return new _DoneStreamSubscription<T>(() => onDone());
             }
 
                 stackTrace = replacement.StackTrace;
             }
 
-            stackTrace ??= AsyncError.defaultStackTrace(error);
+            stackTrace = stackTrace ?? AsyncError.defaultStackTrace(error);
             _sendError(error, stackTrace);
         }
 
         }
 
         void _addPendingEvent(_DelayedEvent<T> evt) {
-            _pending ??= new _StreamImplEvents<T>();
+            _pending = _pending ?? new _StreamImplEvents<T>();
             _pending.add(evt);
         }
 
 
         void addError(object error, string stackTrace) {
             // ArgumentError.checkNotNull(error, "error");
-            stackTrace ??= AsyncError.defaultStackTrace(error);
+            stackTrace = stackTrace ?? AsyncError.defaultStackTrace(error);
             if (!isClosed && _isFiring) {
                 _addPendingEvent(new _DelayedError<T>((Exception) error, stackTrace));
                 return;

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

         }
 
         public void stop() {
-            _stop ??= _now();
+            _stop = _stop ?? _now();
         }
 
         public void reset() {

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

 
             Action startPeriodicTimer = () => {
                 D.assert(timer == null);
-                timer = Timer.periodic(period, (object timer) => {
+                timer = Timer.periodic(period, (object timer1) => {
                     sendEvent();
                     return null;
                 });
                         return;
                     }
 
-                    if (newValue is Future<E> newFuture) {
+                    if (newValue.f is Future<E> newFuture) { // siyao: this if different from dart
                         subscription.pause();
                         newFuture
                             .then(d => add((E) d), onError: (e) => {

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

         _StreamImplEvents<T> _ensurePendingEvents() {
             D.assert(_isInitialState);
             if (!_isAddingStream) {
-                _varData ??= new _StreamImplEvents<T>();
+                _varData = _varData ?? new _StreamImplEvents<T>();
                 return (_StreamImplEvents<T>) _varData;
             }
 
         }
 
         Future _ensureDoneFuture() {
-            _doneFuture ??= _isCanceled ? Future._nullFuture : new _Future();
+            _doneFuture = _doneFuture ?? (_isCanceled ? Future._nullFuture : new _Future());
             return _doneFuture;
         }
 
                 // stackTrace = replacement.stackTrace;
             }
 
-            stackTrace ??= AsyncError.defaultStackTrace(error);
+            stackTrace = stackTrace ?? AsyncError.defaultStackTrace(error);
             _addError(error, stackTrace);
         }
 

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

         // StreamSubscription interface.
 
         public override void onData(Action<T> handleData) {
-            handleData ??= _stream._nullDataHandler;
+            handleData = handleData ?? _stream._nullDataHandler;
             // TODO(floitsch): the return type should be 'void', and the type
             // should be inferred.
             _onData = d => _zone.registerUnaryCallback(data => {
 
         // Siyao: c# does not support convert action
         public override void onError(Action<object, string> handleError) {
-            handleError ??= (input1, input2) => _stream._nullErrorHandler(null);
+            handleError = handleError ?? ((input1, input2) => _stream._nullErrorHandler(null));
 
             _onError = (_, __) => _zone
                 .registerBinaryCallback((in1, in2) => {
         }
 
         public override void onDone(Action handleDone) {
-            handleDone ??= _stream._nullDoneHandler;
+            handleDone = handleDone ?? _stream._nullDoneHandler;
             _onDone = () => _zone.registerCallback(() => {
                 handleDone();
                 return null;
             D.assert(!_inCallback);
             bool wasInputPaused = _isInputPaused;
             _state |= _STATE_IN_CALLBACK;
-            _zone.runUnaryGuarded(data => {
-                _onData((T) data);
+            _zone.runUnaryGuarded(data1 => {
+                _onData((T) data1);
                 return null;
             }, data);
             _state &= ~_STATE_IN_CALLBACK;
                 // TODO(floitsch): this dynamic should be 'void'.
                 var onError = _onError;
                 if (onError != null) {
-                    _zone.runBinaryGuarded((error, stack) => {
-                        onError((Exception) error, (string) stack);
+                    _zone.runBinaryGuarded((error1, stack) => {
+                        onError((Exception) error1, (string) stack);
                         return null;
                     }, error, stackTrace);
                 }
                 return new _DoneStreamSubscription<T>(() => onDone());
             }
 
-            _subscription ??= _source.listen(_controller.add,
+            _subscription = _subscription ?? _source.listen(_controller.add,
                 onError: _controller.addError, onDone: () => _controller.close());
             cancelOnError = Equals(true, cancelOnError);
             return _controller._subscribe(onData, onError, onDone, cancelOnError);

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

             }
 
             if (_handleError != null) {
-                stackTrace ??= AsyncError.defaultStackTrace(error);
+                stackTrace =  stackTrace ?? AsyncError.defaultStackTrace(error);
                 _handleError(error, stackTrace, _sink);
             }
             else {