I agree, Dart offers alot regarding prototyping. Maybe I'm thinking too much, but I believe native os and mobile targets are simply too important in the gaming industry and therefore it's difficult to leverage the device capabilities if your primary target is the web & webgl.
I guess, what I miss most is cross compilation, which would be totally awesome. But Dart moves slowly and I'm very hesitant to invest in a platform where its fate relies on the adoption in browsers. (how long until the Dart VM runs in chrome, ff, ...?)
The dart people seem to be focussed on getting it to generate code that is at fast as handwritten JavaScript. If they get there then you won't have to worry about a dart VM in a browser (won't happen in a non Google browser anyway). You still have really rapid development and rich debugging with the dartium browser (like notch) and you can use a dart VM on the server for faster than js performance.
I guess, what I miss most is cross compilation, which would be totally awesome. But Dart moves slowly and I'm very hesitant to invest in a platform where its fate relies on the adoption in browsers. (how long until the Dart VM runs in chrome, ff, ...?)