[Fiber] Add types for ReactFiber and ReactChildFiber

[koba04]

It might be unnecessary because Flow can infer their return types.
But I think the type annotations for ReactFiber in this is valuable because it's module boundaries.

[gaearon]

Let's get it in, @sebmarkbage can tweak types later if he needs to. 😉

[sebmarkbage]

This is great! On a meta-level though. Currently Fiber isn't properly typed. It has a bunch of any in there. I think that once the code settles a bit we will start creating more specific fiber types such as ClassComponentFiber<T> so that we're safely able to track specific types of fibers and avoid unsound downcasts or any. When that happens, it might be a lot more convenient to rely on the inference rather than adding complex type annotations at every callsite. However, we can add * annotations at that point if we need to. So I don't think we necessarily should make it best practice to always annotate the return value. Even though it may seem like a good practice, it can also create dogma that makes it harder to type things properly.

[koba04]

I got it, that makes sense! Thanks!