But with WebAssembly, you need to DIY a lot and/or send a lot of runtime/stdlib code to the browser. It definitely has its place but something like Dart with excellent stdlib would be amazing (and I know that actually was the intent when Dart was being developed, I know, and I did oppose it at the time). Browsers then could ship different versions, and you can enable them, like "use strict" in js.

On the other hand, I understand why not: You ship Dart and Python folk will be WTF, you ship Python and that would annoy the Java/.NET folk, and so on...