Optimizing JS is interesting, but creating browser-specific applications of code meant for a web audience disturbs me. I don't want a repeat of the first browser wars. Hopefully Mozilla will work toward w3 standards in this effort, since Google clearly hasn't with Dart.
From glancing at the benchmarks in the Ars article, it looks like asm.js generally runs around twice to four times as fast as the same code without asm.js-specific optimizations. This is a huge improvement, but it's not the difference between something being blazing fast in one browser and unusably slow in another.
Additionaly browser games can't have main game loop (cause it would hang the browser infinitely), but often use "requestAnimationFrame" to trigger updates with the best sustained refresh rate available. Even if only one frame out of every 10 in your game is taking 20 ms instead of 16 ms - your game won't run with 60 fps anymore, but with 45 fps or 30 fps because browser is trying to choose refresh rate that will work. And that's a big difference in experience.
With regards to Dart, Google has said that they plan to get the language formally standardized after reaching the stable 1.0 release. It doesn't make sense to try to standardize something that's still in beta.
Mozilla aren't resisting innovation. They've chosen to keep Javascript (the platform) and innovate on top of it, e.g. with pdf.js. Google is aiming to develop alternative platforms. They're both valuable activities.