I would take what kevingadd says with a grain of salt. He's the author of a .NET -> JS compiler (arguably a competitor to Dart) and the experience he cites below is pretty different from what I've had and my colleagues have had. I don't mean that as character assassination - he's obviously a very talented developer - but rather as a point of contrast.

Frankly, you would not want to be touching the compiled JavaScript from Dart - it's not meant to be changed. I agree that if it's that broken, then you should certainly not use the platform. So you should just be writing in JavaScript or just be writing in Dart.