I wish Google would just embrace CoffeeScript instead of this. It's a prettier language, and already solves many of the problems Dart is concerned with.
The primary goal of dart is speed.
What I haven't seen out of the Dart project, so far, is a description of what performance targets they're trying to meet, why they think that no set Javascript extensions (e.g., the "freeze" proposals for Harmony) would suffice to meet those targets, and why they think that any of this is relevant to people whose needs _are_ adequately met by Javascript as it stands.