bump to 4.0.0

+## [4.0.0-nullsafety.0] 
+- Migrate to null-safety 
+- Added ScrollMixin to controllers
+- Added loadingMore status to RxStatus
+- Breaking: It is not possible to initialize Rx with null values.
+
 ## [3.25.6] 
 - Added documentation in French (@kamazoun)
 - Fix logs messages (@damphat)

-import 'package:collection/collection.dart' show IterableExtension;
-
 import '../../../get_core/src/get_main.dart';
 import '../../get_navigation.dart';
 import '../routes/get_route.dart';
@@ -106,3 +104,13 @@ class ParseRouteTree {
     return params;
   }
 }
+
+extension FirstWhereExt<T> on List<T> {
+  /// The first element satisfying [test], or `null` if there are none.
+  T? firstWhereOrNull(bool Function(T element) test) {
+    for (var element in this) {
+      if (test(element)) return element;
+    }
+    return null;
+  }
+}

@@ -11,7 +11,7 @@ dependencies:
     sdk: flutter
 
 dev_dependencies:
-  integration_test:
+  flutter_test:
     sdk: flutter