abstract classes and methods transpile to non-abstract classes and empty methods, respectively

[JeroMiya]

If you take for instance /animations/animation.dart you'll see an abstract dart class Animation<T>. However, the transpiler outputs this as a non-abstract class, and abstract methods are given empty bodies.

This may have been a workaround in the original project for some issue, but if it needs to be reviewed. I'd prefer abstract methods to stay abstract in C#.