Dead code elimination and minification do a good job of reducing Dart2js file size. Dart converted to JavaScript works on IE9 and above and all other modern browsers. JsInterop is done through port send and recieve, so if you need to make a lot of those calls it is potentially slow, but if not, for example I'm making one call to PDF.JS, it works beautifully. Dart's real problem in my opinion is that many 10's (100's?) of nifty JavaScript libs are written everyday that won't work well with Dart. The JavaScript ecosystem just keeps getting better, while the Dart ecosystem plays catchup. Because of this I use TypeScript for some projects.
