remove initial assignment of dispatch in applyMiddleware to make it more expressive

[mz026]

In the original version, the dispatch local variable in applyMiddleware stands for
different things before and after the compose call.
It'd be more expressive to separate "the original one on store" and "the one with middlewares applied"

[aweary]

I think that accessing dispatch as a method of store via store.dispatch is pretty clear, providing the same essential information that storeDispatch does. Is there another advantage of localizing it to storeDispatch?

[mz026]

When first reading the code, I was pretty confused by middlewareAPI.dispatch: why wrapping dispatch into a function that simply returns directly without doing nothing else? That is, why not pass store.dispatch to it like this:

var middlewareAPI = {
  getState: store.getState,
  dispatch: store.dispatch
}

or like this:

var middlewareAPI = {
  getState: store.getState,
  dispatch
}

Then I realized the purpose of wrapping dispatch in the middlewareAPI was that the local variable dispatch changed after this line

dispatch = compose(...chain)(store.dispatch)

so we were actually using the dispatch with middleware in middlewareAPI.dispatch via closure.
However, the assigning statement var dispatch = store.dispatch shadowed this purpose, and by separating them can make this purpose clearer.

[gaearon]

It used to be this way some long time ago. Was changed later to appease Flow but we never merged Flow support anyway so I suppose there's no point to keeping it this way. Let's get this merged.

[gaearon]

FWIW this broke middleware that relied on dispatch during initialization.
We will be deprecating this pattern but for now, I reverted this in #1647.