Gutter icons for running tests should not appear in Dart projects

Is there a reason why Flutter plugin took over running Dart `main` and tests instead of the the Dart plugin?

There are some issues with running dart tests, e.g. [1](https://youtrack.jetbrains.com/issue/WEB-47203) and [2](https://youtrack.jetbrains.com/issue/WEB-55802), and I was going to request support for [test_reflective_loader](https://github.com/dart-lang/test_reflective_loader) test package from the Dart plugin, only to discover that Flutter plugin is actually doing this.

As a user of Flutter, it may not differ much, but since those features are Dart-specific, I find it strange that it isn't implemented in the Dart plugin.

Another point for sure, running some Flutter-specific tests should be handled by the Flutter plugin, with coordination with the Dart plugin in this case.

I guess deciding this point should be made, before trying to fix the issues or proposing new features.