I haven’t tried Dart yet. Just read some of the docs. Can you actually use Dart for any Desktop? How do they do that? OpenGL or kind of like Qt?
As for Machine Learning, you should really check out Julia. It has a clear edge over both Python and Swift. It has a richer ecosystem of math, science and machine learning libraries than Swift. And it runs faster. In fact Julia kicks Fortran, C and C++ to the curb in many real world projects which is why scientists and people doing high performance computing is starting to adopt it. E.g. next generation climate models are being built with Julia due to the high performance and productivity offered.