I was seduced by the Dart language and I would like to build my future webapp in this language, but since only chrome supports to this day this language (and it is unlikely that other browsers will follow) I put all my hopes in dart2js to convert my future application in pure javascript. But I'm afraid I have bad surprises once the work is completed, my question is simple: can I have complete confidence in this tool whatever the target browser is?
Edit1: I understand that the cross-browser compatibility is mainly the job of the developer
Edit2: What I exactly mean is: to what extent can I have confidence that dart2js will convert my entire webapp once finished, that it will not encounter an "internal error" while doing it.
First of all, Chrome does not actually support Dart natively at this point in time (although it is planned for the future). "Unlikely that other browsers will follow" is not necessarily true, it depends on a whole host of factors (e.g. language popularity, performance gains, etc.), but I agree with you that you should only consider dart2js as a viable target at this point.
You can't have complete confidence that dart2js will work exactly the same regardless of browser. But that is the nature of the web currently; different browsers render things differently, have their own bugs and quirks, and in general do not work the same way.
Whether you are developing in Dart and using dart2js, or in Java and using the GWT compiler, or in JavaScript itself, there is no guarantee that the application will work properly across browsers. It is up to you as the developer to test each browser target to verify that it works as expected.
However, since dart2js only supports modern browsers, the discrepancies you will encounter will likely be minor (as opposed to the discrepancies you will discover running an application in latest Chrome vs. IE8 for instance).