But back on topic: I believe Dart was doomed to die as a JS replacement for the sole reason it was not the joint work of all mayor browser vendors, but an attempt of dominance of a single one.
- CoffeeScript came from the JS community (the author also writes underscore, the most popular JavaScript module)
- Many frontend web developers are also Ruby or Python developers
- Typescript came from Microsoft, who use it to do big apps like xbox music. Seperately, C# has beautiful ideas about inline-async via generators that are similar to yield in ES6. They get async, and Typescript is still more popular than Dart.
- Although not browser/DOM related, Swift shows what Apple considers valuable in a language.
- JavaScript now has the largest package manager of any programming language thanks to npm
Whatever succeeds in replacing JavaScript should come from all of those. Dart had a great DOM API, but bared very little resemblance to the languages web developers actually prefer to use.
Dart also has async/await, and synchronous generators using yield.