We have removed the Promise interface from the core libraries. This change is now live in the bleeding_edge branch in source control, and will soon appear in trunk. We encourage you to start migrating to Future now.
I asked Siggi Cherem, engineer on the Dart team, for some background.
Seth: "Future is the future, huh? Sorry I couldn't resist."
Siggi: "Yup. Promise<T> and Future<T> were redundant. We've been migrating all code to use Future, and this change removes the deprecated Promise type."
Seth: "Any differences between Promise and Future?"
Siggi: "The main difference between the two is that in Future we split the code that produces futures, from the code that is used when consuming them. Future<T> is the consumer API, while Completer<T> is the producer API."