Anything happening in a client browser will involve a significant amount of IO - querying / modifying DOM and remote requests. I don't think that choosing Dart over CoffeeScript is going to make much of a difference for most applications.
See Objective-J / Cappuccino which chose to eat the very real overhead messaging in order to gain expressiveness. How many JS apps are as responsive / elegant as 280 North Slides?
I'm referring mainly to the page load time increase, which is important, especially on mobile. (Given what I've seen of the generated code, I suspect runtime performance will be affected too, but I haven't tested, so I can't say for sure.) This is why people create microframeworks—the page load time improvement with e.g. zepto over jQuery is not insignificant.
See Objective-J / Cappuccino which chose to eat the very real overhead messaging in order to gain expressiveness. How many JS apps are as responsive / elegant as 280 North Slides?