Added some free Dart Resources (#7616)

* Added free Dart Resources

* lint: fix resource warnings

- PDF format notes
- extra espaces wrapping authors
- alphabetize listing

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

courses/free-courses-en.md

@@ -325,7 +325,9 @@
 * [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)
+* [Essential Dart](https://www.programming-books.io/essential/dart) - Krzysztof Kowalczyk (HTML)
 * [Free Dart Course](https://www.youtube.com/playlist?list=PL6yRaaP0WPkVR2FiAS7TTCT_n2mDhwISE) - Vandad Nahavandipoor (YouTube)
+* [Learning Dart](https://riptutorial.com/Download/dart.pdf) - Compiled from StackOverflow Documentation (PDF)
 
 
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