I realize this may be a tad premature, and I don't wish to discredit any of the hard work that guys like Alex have put into FlexJS, but a thought occurred to me that perhaps the Web Components spec could/should be a part of our future considerations for FlexJS. With Web Components, we could create Flex "MXML" tags that would already be backed by JavaScript so you would not even need the compiler step. We would even be able to leverage the {} data-binding just like Angular and other JS frameworks do already.
The way I envision it is you write your code in the IDE of your choice, and when you export the release build, you would choose your target just like you do for mobile, and the compiler would only have to translate the AS3 => JS, all the MXML tags would have Web Component analogs that would translate over at least close to 1:1. This may or may not be doable, but it was just a thought that occurred to me. Nick