When the chrome guys started the Sky experiment, now the flutter project, they started with javascript. I don't know why they dropped javascript, but they looked at several languages before trying Dart. Here is a floss weekly episode with one of the co-founders, talks about flutter's hotreload, an idea that came from the Dart team.
https://youtu.be/2C-2-tU6LLY?t=22m19s
If swift was opensource back then they may have gone with it and if they started today they may have gone with kotlin native.
Flutter's hotreload feature, something that they didn't know they wanted, was created after they decided to give Dart a go. While the Dart team had missed the boat on getting the Dart VM into chrome, they still had their internal customers using Dart2JS. The Dart team also had their own experiments in IOT and running on IOS.
Maybe Sky/Flutter guys saw Dart solving their problems that Javascript couldn't and they also saw that the Dart team has the expertise to help them in advancing their experiment to where it is today.
Don't get me wrong. Flutter may well be a formidable piece of software. Dart either. I just don't get how flutter is marketed around the fact that dart was there first with no convincing use case and that people only then started looking for a raison d'etre.