RN: Adopt Mapped Types in EventEmitter

Summary:
Migrates `EventEmitter` in React Native to use the newly introduced mapped types in Flow, instead of `$ObjMap`.

Changelog:
[Internal]

Reviewed By: jbrown215

Differential Revision: D47296240

fbshipit-source-id: 435d0a19242dcd13a4a6c7824daf0e894445cfa7

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

@@ -35,10 +35,9 @@ interface Registration<TArgs> {
   +remove: () => void;
 }
 
-type Registry<TEventToArgsMap: {...}> = $ObjMap<
-  TEventToArgsMap,
-  <TArgs>(TArgs) => Set<Registration<TArgs>>,
->;
+type Registry<TEventToArgsMap: {...}> = {
+  [key in keyof TEventToArgsMap]?: Set<Registration<TEventToArgsMap[key]>>,
+};
 
 /**
  * EventEmitter manages listeners and publishes events to them.
@@ -63,7 +62,7 @@ type Registry<TEventToArgsMap: {...}> = $ObjMap<
 export default class EventEmitter<TEventToArgsMap: {...}>
   implements IEventEmitter<TEventToArgsMap>
 {
-  _registry: Registry<TEventToArgsMap> = {};
+  _registry: Registry<TEventToArgsMap> = emptyRegistry<TEventToArgsMap>();
 
   /**
    * Registers a listener that is called when the supplied event is emitted.
@@ -122,7 +121,7 @@ export default class EventEmitter<TEventToArgsMap: {...}>
     eventType?: ?TEvent,
   ): void {
     if (eventType == null) {
-      this._registry = {};
+      this._registry = emptyRegistry<TEventToArgsMap>();
     } else {
       delete this._registry[eventType];
     }
@@ -137,6 +136,13 @@ export default class EventEmitter<TEventToArgsMap: {...}>
   }
 }
 
+function emptyRegistry<TEventToArgsMap: {...}>(): Registry<TEventToArgsMap> {
+  // Flow cannot enforce that `TEventToArgsMap` is an object because, for
+  // example, Flow permits `empty. We have to ignore this error for now.
+  // $FlowIgnore[incompatible-return]
+  return {};
+}
+
 function allocate<
   TEventToArgsMap: {...},
   TEvent: $Keys<TEventToArgsMap>,