Enable Flutter plug-in for an existing project

[eukreign]

Provide an obvious (for someone who already has experience with intellij) way to enable the flutter plug-in for an existing Flutter project that was not created through Intellij or that does not have an .iml file yet (or has an old one, for example, if a project was created in intellij but before the flutter plugin was installed).

Maybe this could be done via the "Add Framework Support" feature in Intellij?

Or even better, do it just like the Dart plugin does it:

Here: