I tried to ask a question and I got block because I didn’t answer some question.
Here is the answer:
From https://groups.google.com/forum/#!topic/flutter-dev/GO9ilukcoN4 from @Abner Hernandez
First: you need a SQLServerSocket: https://github.com/nippur72/SqlServerSocket is free and works!
Second: you need a client https://github.com/nippur72/SqlServerSocket (look in DartClient folder).
I did several connections and it works very well.
Of course, it is not as "easy to use, with 2 or 3 clicks and less programming..." but yes, you can "implement a simple SELECT * FROM XXX and show the results on both devices (IOS and Android) with Flutter." I did it.!
Yes I tested it with my remote sql and is working on CRUD. I didn't test for extensive query yet.
günter-zöchbauer gives the best practices.
You should use AQUEDUCT aqueduct.io and create a web api so your Flutter app connects to the server more securely.