For Flutter to work, you need to have the Dart-SDK as well. A lot of developers now uses, Flutter, Dart, AngularDart and all the tools related to it. Should we add a request for other packages? or all can be done under this one? but in my opinion you don't need this in the repo. You just download the tar.gz file and extract it in /opt/

No, we won't have to ship Dart with Flutter. Flutter uses it's own precompiled version of Dart. So a Flutter package would just be the packaged version of the release tarball. It is not a very clean way of distributing an open source package, but Google decided to do it that way.
I still think we should have these packages in the repository. Solus is an operating system where I would be confident to give it someone who is not a Linux nor a command line expert, someone who maybe has never used tar before. For those people it would be ideal to do the obvious: Go to the software center, search for Flutter and install it.

Well, you can actually use flutter without any command line at all. And honestly the biggest upside is not having to copy files on every update manually if one can simply update the package.
Anyway, I will try to get Dart working and then we can look further into flutter.

Maybe if you hide behind an IDE that does it all for you. But the fact of the matter is we don't include every package under the sun that's used for development. There have to be limits or we'd spend all of our time doing updates and none of our time on solus projects.

Sure but until then this stays closed.