Add some check on EmitterSubscription's listener

[em-yuanbo]

https://github.com/facebook/emitter/blob/master/src/EmitterSubscription.js#L32
there is no validation on the param listener

  constructor(subscriber: EventSubscriptionVendor, listener, context: ?Object) {
    super(subscriber);
    this.listener = listener;
    this.context = context;
  }

and no appliable check when apply the listener.
https://github.com/facebook/emitter/blob/master/src/BaseEventEmitter.js#L182

  __emitToSubscription(subscription, eventType) {
    var args = Array.prototype.slice.call(arguments, 2);
    subscription.listener.apply(subscription.context, args);
  }

can we add some checks or make some noises when create a new EmitterSubscription?

[kyldvs]

The file has typechecks which will complain if you pass anything other than a function to it. We should also probably add flow types too, but I do not believe we should modify runtime behavior to verify it's a function.

[em-yuanbo]

The typechecks or the flow types only make sense when you compile this source code. But most time the libs we used are installed from npm and compiled, and all the types or checks are missed. by then the condition is different.

[kyldvs]

Yeah that will be true of typechecks probably, but for flow we can provide external library definitions. Doesn't matter if the module is transpiled or not