To clarify, by "suboptimal" I meant that the Javascript code generated by transpiling a Dart application will almost certainly be less optimal than writing that

To clarify, by "suboptimal" I meant that the Javascript code generated by transpiling a Dart application will almost certainly be less optimal than writing that application in Javascript from the outset. My point is that this is not really true. The Dart language itself is designed to compile to efficient Javascript. This is one reason it's a better solution than GWT in my opinion, because Java does not always compile to efficient Javascript and you do have to skip features like longs, Java regexes, even the java.util collections classes are heavyweight. Dart being designed from the start to compile to Javascript avoids most of these problems. Like CoffeeScript, I expect Dart performance to be very close to Javascript when compiled, sometimes even faster than what a programmer would normally write.