Serious answer:
The Dart2js compiler could easily take advantage of asm.js additions, and generate javascript code that uses asm.js. If asm.js does take off, all it can do is help Dart generate faster running code.
Now, if we are talking about the DartVM, asm.js could be used as an argument not to include the DartVM in other browsers. But asm.js is still very much in it's infancy, so we really need to wait and see how it's adopted and how it progresses.