Flux: async communication with websockets
ActionCreator.js


Why write a class for when you just export a single instance? When one makes a class, one expects to be able to make multiple instances. This doesn't. You can just roll with object literals. You could even just write functions and just export them.
service is undefined. Where is it?


todo-service.js


Same as above, why write a class?


WebSocketWrapper.js


Same as above, why write a class? Even more interesting is that you have "tracking variables" (reqId and responseDeffers) outside the class.
Not sure how you'd benefit with let in you code. You're declaring them at the very top of a function scope. var would have sufficed.


Just because JS has ES6 doesn't mean you have to use classes, let and all the new stuff.