I hope Google isn't going to solely rely on Chrome having a Dart VM to increase Dart performance. As it stands; no other browser is interested in the Dart VM, so your app will only benefit from the speed in Chrome while other browsers are at the mercy of the efficiency of the Dart->JS compiler. If the compiler can produce near-JS performance I wish them and Dart developers well, otherwise using Dart will only handicap you in other browsers. Chrome has a respectable market share on the desktop, but mobile will be dominated by Safari and Android's stock browsers for at least another year (Chrome-Android is ICS only); thus you're at a complete disadvantage on mobile.
If Google wanted a second language on Android, they'd have it by now. Either golang or JS+V8 (possibly out-of-browser) would have been fine choices. As well, Dart's stated goal is "structured web programming", and parts of it's design are constrained by the need to run within the browser and compile down to JS. Imposing those same restrictions on Android development would be counterproductive.
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.
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.
