Should FSA mandate actions to be serializable? I think it's a good requirement (prevents people from passing callbacks inside actions). It's also great for the tooling, as it's easy to build Flux over websockets, or a persistent debug session solution, without worrying that something might get lost.

I propose the following changes:

• invariant(isDeepEqual(action, JSON.parse(JSON.stringify(action))))
• Therefore,type must be a String, not a Symbol

We could then write a middleware that checks actions for compliance in development.