You can read its short wiki for some clues: https://github.com/dart-lang/fletch/wiki/Processes-and-Isola...
I like Fletch's idea very much. Imagine not having to worry about Async all the time.
Not sure how everything is implemented in Fletch, but I think I heard that in Fletch they are able to share 1 thread per many processes if need be. And they have been trying hard to save memory while implementing those features.
If you want to run some tests to compare with, I created some small samples using different Dart implementations and NodeJS here: https://github.com/jpedrosa/arpoador/tree/master/direct_test...
Fletch also supports a kind of Coroutine: https://github.com/dart-lang/fletch/wiki/Coroutines-and-Thre...