JS has improved a lot in 17 years. It's still incumbent, and with ES6 and further Harmony work, it is still improving (much of ES6 are in V8 and SpiderMonkey, or coming very soon).
Good luck displacing JS. I mean that sincerely, especially if you work in the open. Dart had a rough start but even ignoring that, I don't think it will succeed. We'll see.
This looks like a good place to fulfill a promise I made to report back if my hopes were dashed about Dart (http://news.ycombinator.com/item?id=2989686). They were dashed pretty quickly. I've seen nothing compelling enough to prefer Dart to a fixed-as-much-as-is-practical JS. Especially if such a JS becomes more suitable as a compilation target (better numerics and so on).
Good luck displacing JS. I mean that sincerely, especially if you work in the open. Dart had a rough start but even ignoring that, I don't think it will succeed. We'll see.