Separate current owner and dispatcher

[bvaughn]

This is part tech-debt cleanup, since the dispatcher didn't really belong in the owner ref to begin with.

Also this will be used by the DevTools to support hooks. DevTools need a way to modify the current dispatcher, so renderers will need to inject this ref at runtime.

Note that external libraries depending on React internals may be impacted by this, but there are migration strategies involving feature detection and in general, we don't really support accessing the internals data structure.

[TrySound]

Could you mention this in changelog? Me and probably somebody else use currentOwner to read from context since v16.6

[bvaughn]

Sure. We can add an entry to the changelog about this, since it impacts libraries like Relay.

In Relay's case, they would need to change this to something more like this:

const React = require("React");

const {ReactCurrentDispatcher, ReactCurrentOwner}
  = React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED;

function readContext<T>(Context: React.Context<T>): T {
  const dispatcher =
    ReactCurrentDispatcher != null
      ? ReactCurrentDispatcher.current
      : ReactCurrentOwner.currentDispatcher;
  return dispatcher.readContext(Context);
}

module.exports = readContext;

cc @kassens