To piles of JavaScript, you mean.
As well as, forcing developers to infer the way Dart compiler generates JavaScript code, in the absence of source maps.
I rather use JavaScript, thank you.
dart2js generates source maps
You need browser support for them.
Use chrome when debugging
How to debug browser specific behaviors?
I believe others (at least Firefox) will add eventually. https://wiki.mozilla.org/DevTools/Features/SourceMap
When it might matter is if there are apps (like games) that run faster in a Dart VM than in javascript. Anyway though, even Chrome doesn't include a Dart VM yet by default, so no big deal yet.