No it couldn't. asm.js has no garbage collector. The only way to get dart running on asm.js would be to port the entire VM to asm.js, which afiak won't work since the dart JIT emits machine code at runtime.
I don't think @kyrra was suggesting that Dart would compile completely to asm.js code, just parts where it makes sense (just like normal JavaScript uses asm.js only where it makes sense).