[spankalee]

One of Dart's design goals is to be able to be compiled to efficient Javascript. That does limit some of the choices that the language designers can make, but it means that there should be good performance on all modern browsers.

The VM presumably takes advantage of the slightly less dynamic nature of Dart to make more agressive optimizations than V8.

[ConstantineXVI]

I'm just concerned that Google (being both the origin of Dart and a prominent developer of webapps) will spend most of their development time writing Dart apps running on their Dart VM and slowly begin to neglect (remember, doesn't necessarily have to be malicious) the performance of their JS output.