The JS output that gets generated from a Dart program isn't just "dart_size * multiplier", it's "dart_size * multiplier + constant". So, when you see a tiny Dart program being compiled to a large amount of JS, that doesn't mean a large Dart program will compile to a huge amount of JS, it just means that the constant is dwarfing the multiplier.

The constant is coming from the runtime library (or, more accurately, the portions of the runtime library that your program uses). That's relatively fixed so as your Dart program gets larger, the overhead of that becomes relatively smaller. The actual code-size multiplier when going from Dart -> JavaScript is actually fairly small and getting smaller over time.

In some cases, we may actually compile Dart code to less JavaScript code because of things like dead code elimination and constant folding.

JavaScript code size is something that we consider a core metric for dart2js and we're constantly working to reduce it.

hahaha, i totally agree - if it is 50x larger i would use a different tool too :) But as i said, if your source is 100KB it's not 50x larger, maybe it's 20% larger than hand written JS. Currently the footprint of the js-code is around 150KB. While dart2js is evolving this number is declining constantly. In the end when you add js-minification you will have a footprint of maybe 20-30KB i guess.