refactor: Added some missing overrides and ignored a null warning

@@ -504,6 +504,7 @@ class _CupertinoBackGestureDetectorState<T>
     }
     }
     // FIXME: shouldn't we return a default here?
     // FIXME: shouldn't we return a default here?
     //  or perhaps throw error
     //  or perhaps throw error
+    // ignore: avoid_returning_null
     return null;
     return null;
   }
   }
 
 

@@ -28,7 +28,9 @@ class RxList<E> implements List<E>, RxInterface<List<E>> {
   @override
   @override
   bool get isNotEmpty => value.isNotEmpty;
   bool get isNotEmpty => value.isNotEmpty;
 
 
-  StreamController<List<E>> subject = StreamController<List<E>>.broadcast();
+  @override
+  StreamController<List<E>> subject = StreamController.broadcast();
+
   final Map<Stream<List<E>>, StreamSubscription> _subscriptions = {};
   final Map<Stream<List<E>>, StreamSubscription> _subscriptions = {};
 
 
   void operator []=(int index, E val) {
   void operator []=(int index, E val) {
@@ -188,9 +190,7 @@ class RxList<E> implements List<E>, RxInterface<List<E>> {
     if (_subscriptions.containsKey(rxGetX)) {
     if (_subscriptions.containsKey(rxGetX)) {
       return;
       return;
     }
     }
-    _subscriptions[rxGetX] = rxGetX.listen((data) {
-      subject.add(data);
-    });
+    _subscriptions[rxGetX] = rxGetX.listen(subject.add);
   }
   }
 
 
   set value(List<E> val) {
   set value(List<E> val) {

@@ -56,6 +56,7 @@ class RxSet<E> implements Set<E>, RxInterface<Set<E>> {
     return val;
     return val;
   }
   }
 
 
+  @override
   void addAll(Iterable<E> item) {
   void addAll(Iterable<E> item) {
     _set.addAll(item);
     _set.addAll(item);
     subject.add(_set);
     subject.add(_set);