move multistream to a single file as it is not supported in the original flutter 1.17.5

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

                 () => (_PendingEvents<T>) new _IterablePendingEvents<T>(elements));
         }
 
-        public static Stream<T> multi(Action<MultiStreamController<T>> onListen, bool isBroadcast = false) {
-            return new _MultiStream<T>(onListen, isBroadcast);
-        }
-
         public static Stream<T> periodic(TimeSpan period,
             Func<int, T> computation = null) {
             Timer timer = default;
             _sink.close();
             return Future._nullFuture;
         }
-    }
-
-    public interface MultiStreamController<T> : IStreamController<T> {
-        void addSync(T value);
-
-        void addErrorSync(object error, string trackStack);
-
-        void closeSync();
     }
 }

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

             return new _DoneStreamSubscription<T>(() => onDone());
         }
     }
-
-    class _MultiStream<T> : Stream<T> {
-        public override bool isBroadcast {
-            get {
-                return _isBroadcast;
-            }
-        }
-
-        bool _isBroadcast;
-
-        /// The callback called for each listen.
-        public readonly Action<MultiStreamController<T>> _onListen;
-
-        public _MultiStream(Action<MultiStreamController<T>> _onListen, bool isBroadcast) {
-            _isBroadcast = isBroadcast;
-            this._onListen = _onListen;
-        }
-
-        public override StreamSubscription<T> listen(Action<T> onData, Action<object, string> onError = null,
-            Action onDone = null, bool cancelOnError = false) {
-            var controller = new _MultiStreamController<T>();
-            controller.onListen = () => {
-                _onListen(controller);
-            };
-            return controller._subscribe(
-                onData, onError, onDone, cancelOnError);
-        }
-    }
-
-    class _MultiStreamController<T> : _AsyncStreamController<T>, MultiStreamController<T> {
-        public _MultiStreamController() : base(null, null, null, null)
-        {
-        }
-
-        public void addSync(T value) {
-            if (!_mayAddEvent) throw _badEventState();
-            if (hasListener) _subscription._add(value);
-        }
-
-        public void addErrorSync(object error, string trackStack) {
-            if (!_mayAddEvent) throw _badEventState();
-            if (hasListener) {
-                _subscription._addError(error, trackStack ?? "");
-            }
-        }
-
-        public void closeSync() {
-            if (isClosed) return;
-            if (!_mayAddEvent) throw _badEventState();
-            _state |= _StreamController<T>._STATE_CLOSED;
-            if (hasListener) _subscription._close();
-        }
-
-        public override Stream<T> stream {
-            get {
-                throw new Exception("Not available");
-            }
-        }
-    }
 }

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

+using System;
+
+namespace Unity.UIWidgets.async {
+    public class StreamMultiUtils<T>
+    {
+        public static Stream<T> multi(Action<MultiStreamController<T>> onListen, bool isBroadcast = false) {
+            return new _MultiStream<T>(onListen, isBroadcast);
+        }
+    }
+    
+    public interface MultiStreamController<T> : IStreamController<T> {
+        void addSync(T value);
+
+        void addErrorSync(object error, string trackStack);
+
+        void closeSync();
+    }
+    
+    class _MultiStream<T> : Stream<T> {
+        public override bool isBroadcast {
+            get {
+                return _isBroadcast;
+            }
+        }
+
+        bool _isBroadcast;
+
+        /// The callback called for each listen.
+        public readonly Action<MultiStreamController<T>> _onListen;
+
+        public _MultiStream(Action<MultiStreamController<T>> _onListen, bool isBroadcast) {
+            _isBroadcast = isBroadcast;
+            this._onListen = _onListen;
+        }
+
+        public override StreamSubscription<T> listen(Action<T> onData, Action<object, string> onError = null,
+            Action onDone = null, bool cancelOnError = false) {
+            var controller = new _MultiStreamController<T>();
+            controller.onListen = () => {
+                _onListen(controller);
+            };
+            return controller._subscribe(
+                onData, onError, onDone, cancelOnError);
+        }
+    }
+
+    class _MultiStreamController<T> : _AsyncStreamController<T>, MultiStreamController<T> {
+        public _MultiStreamController() : base(null, null, null, null)
+        {
+        }
+
+        public void addSync(T value) {
+            if (!_mayAddEvent) throw _badEventState();
+            if (hasListener) _subscription._add(value);
+        }
+
+        public void addErrorSync(object error, string trackStack) {
+            if (!_mayAddEvent) throw _badEventState();
+            if (hasListener) {
+                _subscription._addError(error, trackStack ?? "");
+            }
+        }
+
+        public void closeSync() {
+            if (isClosed) return;
+            if (!_mayAddEvent) throw _badEventState();
+            _state |= _StreamController<T>._STATE_CLOSED;
+            if (hasListener) _subscription._close();
+        }
+
+        public override Stream<T> stream {
+            get {
+                throw new Exception("Not available");
+            }
+        }
+    }
+}

com.unity.uiwidgets/Runtime/async/stream_multi.cs.meta

+fileFormatVersion: 2
+guid: 5bf5bf2fe1c7407db0a476f2bbcd01f4
+timeCreated: 1629701152