Remove special case for `invariant`

I'm finding it difficult to add type annotations to my function named `invariant()` because of this little guy: https://github.com/facebook/flow/blob/0f56d1742967b855ab5ee5eaee93d2edfc503e54/src/typing/type_inference_js.ml#L2058

It seems that many of these functions are special cased. I'm curious as to why, because I can't use any functions that share these names anymore.