Think of it the other way around: Dart was created for the sake of Flutter.

You seem to think that the framework designers at Google just decided to pick a niche language called Dart to write Flutter in when they could have picked Javascript to write Flutter in. No. They decided to make the design of Flutter as a framework so clean and pure that they would have to work backwards to figure out what language it would need to be written in and then they created Dart.

At least that's how I like to imagine it.