Looking at the slides, I don't think this is accurate. You can compile Dart into Javascript, which will run on non-Chrome browsers.

The advantage is that (1) on Chrome, you get better performance, and (2) you get a nicer language that Javascript.

Dart is presently broken by design for web browsers in general, because they have to ship a set of type mapping tables that contains names & information specific to individual browser versions. In the past, new releases of Firefox have broken it and those errors had to be pointed out to the dev team for a later fix.

Ideally in the future the Dart team will be more on top of things, but you will still have to recompile your Dart applications every time a new browser comes out. Many developers will probably not remember to do this - if they're maintaining their apps at all.

Dart2js is also an afterthought (in general) compared to the mainline dart vm, so it will be possible (if not common) for dart applications to end up only being compatible with Chrome's implementation. One of the main issues I'm aware of in this area is that integers larger than 53-bit will silently fail to work correctly, while native Dart will handle them.

And now I have to support (and debug) two languages only because on a single browser one can perform slightly better, while the rest of the resources and frameworks for web clients are all written in javascript. No thanks

Why would you be supporting two languages? You can choose to write your app in JavaScript (that's not going away and that's one language) or you can choose to write your app in Dart (also just one language) which will run as Dart on the Dart VM and compiled-you-don't-touch-it JavaScript everywhere else. Sounds like FUD to me.

Because if I choose dart I have still to deal with the derived javascript code implementation I must use to run in other browser that are not chrome and, as described by the post close to this, often that js is so broken it cause big bugs on firefox for example, and to understand and repair those you have to know js.

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.