Funny thing, it has been years since Dart started, ditto PNaCl. Who says JS is the slow path? I suspect it will get there faster (for tangible definitions of "there", e.g., a Rust2JS compiler for you) with well-focused work.
Pretty please! Excited to see pcwalton's zero.rs work, which I understand to be a prerequisite for a Rust2JS. But deeper JS integration than emscripten currently provides (e.g., mapping rust structs to harmony binary data StructTypes?) would be even more awesome. When are you going to land 749786, anyway?
Re: Dart and PNaCl--they've been useful for providing political cover to people who want to evolve JS more aggressively.
In theory it shouldn't be necessary to mal-invest in long-odds/single-vendor (and therefore hard to standardize without dominant market power) innovations at high cost, just to drive minimal investment in evolving the web in shorter-odds ways. Especially when the same company is collaborating in TC39 to evolve JS, and owns the V8 engine!