1. WebKit rejected adding barriers to support multiple garbage collectors and a super-GC to collect cycles, required by Dart or any 2nd VM. See https://lists.webkit.org/pipermail/webkit-dev/2011-December/... where Fil Pizlo of Apple cites 5% throughput regression.

Dart was never going to make it, and adding 3 or 4 VMs with their own GCs is even less plausible.

2. See https://github.com/WebAssembly for progress on what asm.js showed was possible: a 2nd, initially co-expressive with asm.js, eventually more expressive, binary syntax for the Web. It's happening now, this year and probably shipping experimentally in 1Q2017.

Writing "easily take a decade" out of ignorance just says to me you didn't even bother to use google!