RN: Adopt Indexed Access Types in EventEmitter (facebook#38243)

Summary:
Pull Request resolved: facebook#38243

Adopts [Indexed Access Types](https://flow.org/en/docs/types/indexed-access/) in `EventEmitter`.

Changelog:
[Internal]

Reviewed By: gkz, SamChou19815

Differential Revision: D47298095

fbshipit-source-id: d31eae8a929a13caf8d876aed4a035982f52ab38

packages/react-native/Libraries/vendor/emitter/EventEmitter.js

@@ -15,13 +15,13 @@ export interface EventSubscription {
 export interface IEventEmitter<TEventToArgsMap: {...}> {
   addListener<TEvent: $Keys<TEventToArgsMap>>(
     eventType: TEvent,
-    listener: (...args: $ElementType<TEventToArgsMap, TEvent>) => mixed,
+    listener: (...args: TEventToArgsMap[TEvent]) => mixed,
     context?: mixed,
   ): EventSubscription;
 
   emit<TEvent: $Keys<TEventToArgsMap>>(
     eventType: TEvent,
-    ...args: $ElementType<TEventToArgsMap, TEvent>
+    ...args: TEventToArgsMap[TEvent]
   ): void;
 
   removeAllListeners<TEvent: $Keys<TEventToArgsMap>>(eventType?: ?TEvent): void;
@@ -71,19 +71,19 @@ export default class EventEmitter<TEventToArgsMap: {...}>
    */
   addListener<TEvent: $Keys<TEventToArgsMap>>(
     eventType: TEvent,
-    listener: (...args: $ElementType<TEventToArgsMap, TEvent>) => mixed,
+    listener: (...args: TEventToArgsMap[TEvent]) => mixed,
     context: mixed,
   ): EventSubscription {
     if (typeof listener !== 'function') {
       throw new TypeError(
         'EventEmitter.addListener(...): 2nd argument must be a function.',
       );
     }
-    const registrations = allocate<_, _, TEventToArgsMap[TEvent]>(
+    const registrations = allocate<TEventToArgsMap, TEvent, TEventToArgsMap[TEvent]>(
       this._registry,
       eventType,
     );
-    const registration: Registration<$ElementType<TEventToArgsMap, TEvent>> = {
+    const registration: Registration<TEventToArgsMap[TEvent]> = {
       context,
       listener,
       remove(): void {
@@ -103,10 +103,10 @@ export default class EventEmitter<TEventToArgsMap: {...}>
    */
   emit<TEvent: $Keys<TEventToArgsMap>>(
     eventType: TEvent,
-    ...args: $ElementType<TEventToArgsMap, TEvent>
+    ...args: TEventToArgsMap[TEvent]
   ): void {
     const registrations: ?Set<
-      Registration<$ElementType<TEventToArgsMap, TEvent>>,
+      Registration<TEventToArgsMap[TEvent]>,
     > = this._registry[eventType];
     if (registrations != null) {
       for (const registration of [...registrations]) {
@@ -140,7 +140,7 @@ export default class EventEmitter<TEventToArgsMap: {...}>
 function allocate<
   TEventToArgsMap: {...},
   TEvent: $Keys<TEventToArgsMap>,
-  TEventArgs: $ElementType<TEventToArgsMap, TEvent>,
+  TEventArgs: TEventToArgsMap[TEvent],
 >(
   registry: Registry<TEventToArgsMap>,
   eventType: TEvent,