The plus side is that Google can develop Dart alongside the framework. This allows some nice things like excellent hot loading support, fast AOT compilation for release builds, a GC tuned for UI work, etc that would be harder or even impossible in a general purpose language with different goals.
It's also a pretty easy language to learn if you're already familiar with any other modern mainstream language.
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.
Dart pre-dates Flutter by a few years and had failed (a couple of times) as a web development language before Flutter chose to employ it. Would Dart be around today, outside of Google if not for Flutter? Seems unlikely.
Flutter indeed did decide to pick a niche language, albeit one that was used internally at Google. Tim Sneath discusses it on Software Engineering Radio:
As far as I know, Dart was created beforehand in order to "replace" Javascript. That did never work out, it never got any real traction, so Dart eventually stumbled around without any real purpose. Then Flutter came, used Dart (because they needed some language like JS, but not JS, and Dart was "there" and could be used because it was a Google product) and then Flutter started to gain traction, and so did Dart, but only as a side effect of Flutter now being the only real "user" of the language.
