added a few Dart and Flutter online courses (#6877)

courses/free-courses-en.md

@@ -303,6 +303,7 @@
 * [Dart Programming in 4 hours \| Full beginners tutorial](https://www.youtube.com/watch?v=5xlVP04905w) - Mike Dane (YouTube)
 * [Dart Programming Tutorial - Full Course](https://www.youtube.com/watch?v=Ej_Pcr4uC2Q) - Mahmud Ahsan (FreeCodeCamp)
 * [Dart Programming Tutorial \| Learn the Dart for Flutter](https://www.udemy.com/course/dart-programming-tutorial-learn-the-dart-for-flutter) (Udemy)
+* [Free Dart Course](https://www.youtube.com/playlist?list=PL6yRaaP0WPkVR2FiAS7TTCT_n2mDhwISE) - Vandad Nahavandipoor (YouTube)
 
 
 ### Data Science
@@ -390,8 +391,10 @@
 ### Flutter
 
 * [Flutter Crash Course](https://fluttercrashcourse.com/courses/basics/lessons/materialapp-scaffold-appbar-text) - Nick Manning *(account required)*
+* [Flutter State Management Course](https://www.youtube.com/playlist?list=PL6yRaaP0WPkUf-ff1OX99DVSL1cynLHxO) - Vandad Nahavandipoor (YouTube)
 * [Flutter Tutorial](https://www.udacity.com/course/build-native-mobile-apps-with-flutter--ud905) - Matt Sullivan, James Williams, Mary Xia (Udacity)
 * [Flutter Tutorial for Beginners](https://www.udemy.com/course/learn-flutter-beginners-course) - Mayuresh Wankhede (Udemy)
+* [Free Flutter Course](https://www.youtube.com/playlist?list=PL6yRaaP0WPkVtoeNIGqILtRAgd3h2CNpT) - Vandad Nahavandipoor (YouTube)
 
 
 ### Game Development