(2) Chrome ends up with 90%+ market share, so it doesn't matter that I'm bad in all the other browsers. But I can't see this happening as long as e.g. iOS is around and relevant.
(3) I have the resources to write both a Dart version for Chrome and a CoffeeScript version for older browsers. At this point, Google has strictly made my life harder than it would have been otherwise.
I do want to give Dart a fair shake, but I'm having a hard time seeing how it could realistically succeed.
But will it, though? Sure, "Hello, World" transpiles to 17,000 lines of code, but I'd want to see some actual benchmarks of actual code to see what kind of performance hit that implies, before deciding against Dart on that basis.
And then there's the whole python argument about developer time being more valuable, and most applications these days not being performance constrained, etc, that could possibly apply here, as well.
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?