Oh. Dear. God. JavaScript is going to become the x86 of our time, i.e., the historically awful patchwork of kludges that gets passed onto each generation to kludge anew which also runs the entire world. JavaScript: we don't have integers, but, dammit, we have vector instructions. FML.
It looks like Alon Zakai is working on something the "Emterpreter," which is a bytecode format that Emscripten could compile to to sacrifice some runtime performance for startup time. (JS parse time is quite a big deal for Emscripten applications)
I am hoping that this effort goes really really well. So well that browser engines start natively supporting this bytecode.
Apologies, but we don't push Dart. You don't have to use it. Google writes millions of lines of JavaScript a year and wants a better language, that's all.
As pcwalton says, SIMD.js evens an advantage that was in favor of Dart, while at the same time making JavaScript a better compile target for languages like Dart and asm.js languages like C++. It's also a really clean API that fits nicely onto typed arrays. What's not to like?
Maybe it's not too late to take up painting.