Update 102-flutter.md

Add links with more information about Flutter and tutorials

content/roadmaps/100-frontend/content/121-mobile-applications/102-flutter.md

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-# Flutter
+# Flutter
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+Flutter is a free and open-source mobile UI framework created by Google and released in May 2017. In a few words, it allows you to create a native mobile application with only one codebase. This means that you can use one programming language and one codebase to create two different apps (for iOS and Android).
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+Flutter consists of two important parts:
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+- An SDK (Software Development Kit): A collection of tools that are going to help you develop your applications. This includes tools to compile your code into native machine code (code for iOS and Android).
+- A Framework (UI Library based on widgets): A collection of reusable UI elements (buttons, text inputs, sliders, and so on) that you can personalize for your own needs.
+To develop with Flutter, you will use a programming language called Dart. The language was created by Google in October 2011, but it has improved a lot over these past years.
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+Dart focuses on front-end development, and you can use it to create mobile and web applications.
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+If you know a bit of programming, Dart is a typed object programming language. You can compare Dart's syntax to JavaScript.
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+- Flutter Webpage: https://flutter.dev/
+- Tutorial: https://docs.flutter.dev/reference/tutorials
+- Flutter Tutorial: https://www.w3adda.com/flutter-tutorial
+- Flutter Tutorial for Beginners: https://www.youtube.com/watch?v=1ukSR1GRtMU&list=PL4cUxeGkcC9jLYyp2Aoh6hcWuxFDX6PBJ
+- Dart Tutorial: https://www.tutorialspoint.com/dart_programming/index.htm