Dart is awesome. Just because currently the community is still small doesn't mean much. Google is working on so much great stuff that Dart becomes more compelling every month. They are lying low since awhile with public announcements, but if you follow the commits there is so much work being done. In a few months it will become hard to ignore Dart.

The combined development (TS+Dart) was slowing Angular development down and contribution was painful because one needed knowledge of both languages. There are notable downsides for the split development but I fully understand the decision. Implementing everything in a way that it can be translated into the other language was a real pain and limited both versions to a common denominator which prevented both versions to benefit from the advantages of their respective language.