not really, if you're writing flash apps, there's a huge chance that you're actually writing as3 code, which gets compiled into DoABC tags (actionscript bytecode) and I don't think DoABC tags will ever be supported unless they implement a stack-based virtual machine in javascript.

It gives the appearance that it got cheaper to re-target, but really it will be a ton more work when you realize what will never be supported. Even if they managed to implement a virtual machine in javascript, they'd still need to implement all the library code that normally doesn't ship with every swf file.