V8 supports asm.js as any other ES3-compliant JavaScript engine. It also has many optimizations in place which benefit from asm.js-style code. Asm.js-style code also simply doesn't do those expensive JavaScript-y things. That's why that Citadel demo runs just fine in Chrome, too.

However, V8 doesn't have a separate mode. The "'use asm';" pragma is ignored.

The Octane benchmark suite does contain Emscripten and Mandreel benchmarks though. So, they are tracking it and they also try to make this kind of code run faster.