In which browser?

None, until SIMD appears in JavaScript.

Which was the whole point of my question.

Dart has become just another language that transpiles to JavaScript, abandoned even by Googlers.

Google use it for the most mission-critical part of their business, I'm not sure it means "abandoned". http://news.dartlang.org/2015/03/dart-for-entire-web.html

And more: https://www.dartlang.org/community/who-uses-dart.html

That is not the image the Angular team has shown to the world.

What is the business value to use Dart instead of any other language that transpiles into JavaScript?

Angular 2 exists in 3 forms: TypeScript, Dart and ES5. And a lot of ideas came to ng2 from AngularDart.

> What is the business value to use Dart instead of any other language that transpiles into JavaScript?

It's the question of preferences. Dart gives great tooling (IDE, debugging, testing) and more advantaged language, but universe of Dart is much smaller, so there are pros and cons, as always. There is no one ideal size of shoes.

My understanding is that Dart is being used directly, just not in any mainstream browsers. I believe Google Fiber uses Dart for their set-top boxes, but there is also use internally at the company (i.e. intranet).

All details are in link, but tl;dr is "Not in browsers yet".