fix(database): react-native 0.73 compatibility

upstream EventEmitter made registrations fully private, and we
were relying on them to implement specific-listener-removal to
meet a firebase-js-sdk API contract

implemented listener registration accounting in situ in place of
previous style relying on upstream EventEmitter implementation

packages/database/lib/DatabaseSyncTree.js

@@ -26,13 +26,32 @@ class DatabaseSyncTree {
     this._tree = {};
     this._reverseLookup = {};
 
+    // we need to be able to register multiple listeners for a single event,
+    // *then delete a specific single listener for that event*, so we have to
+    // be able to identify specific listeners for removal which means we need
+    // to mirror the private registration accounting from upstream EventEmitter
+    // so we have access here and can do our single emitter removal
+    // This is a map of emitter-key <-> set of listener registrations
+    // The listener registrations have { context, listener, remove() }
+    this._registry = {};
+
     SharedEventEmitter.addListener('database_sync_event', this._handleSyncEvent.bind(this));
   }
 
   get native() {
     return NativeModules.RNFBDatabaseQueryModule;
   }
 
+  // from upstream EventEmitter: initialize registrations for an emitter key
+  _allocate(registry, eventType) {
+    let registrations = registry[eventType];
+    if (registrations == null) {
+      registrations = new Set();
+      registry[eventType] = registrations;
+    }
+    return registrations;
+  }
+
   /**
    * Handles an incoming event from native
    * @param event
@@ -132,6 +151,13 @@ class DatabaseSyncTree {
     if (isString(registrations)) {
       this.removeRegistration(registrations);
       SharedEventEmitter.removeAllListeners(registrations);
+
+      // mirror upstream accounting - clear out all registrations for this key
+      if (registrations == null) {
+        this._registry = {};
+      } else {
+        delete this._registry[registrations];
+      }
       return 1;
     }
 
@@ -141,6 +167,12 @@ class DatabaseSyncTree {
     for (let i = 0, len = registrations.length; i < len; i++) {
       this.removeRegistration(registrations[i]);
       SharedEventEmitter.removeAllListeners(registrations[i]);
+      // mirror upstream accounting - clear out all registrations for this key
+      if (registrations[i] == null) {
+        this._registry = {};
+      } else {
+        delete this._registry[registrations[i]];
+      }
     }
 
     return registrations.length;
@@ -163,15 +195,10 @@ class DatabaseSyncTree {
       const registration = registrations[i];
       let subscriptions;
 
-      // EventEmitter in react-native < 0.70 had a `_subscriber` property with a method for subscriptions by type...
-      if (SharedEventEmitter._subscriber) {
-        subscriptions = SharedEventEmitter._subscriber.getSubscriptionsForType(registration);
-      } else {
-        // ...react-native 0.70 now stores subscribers as a map of Sets by type in `_registry`
-        const registrySubscriptionsSet = SharedEventEmitter._registry[registration];
-        if (registrySubscriptionsSet) {
-          subscriptions = Array.from(registrySubscriptionsSet);
-        }
+      // get all registrations for this key so we can find our specific listener
+      const registrySubscriptionsSet = this._registry[registration];
+      if (registrySubscriptionsSet) {
+        subscriptions = Array.from(registrySubscriptionsSet);
       }
 
       if (subscriptions) {
@@ -180,6 +207,7 @@ class DatabaseSyncTree {
           // The subscription may have been removed during this event loop.
           // its listener matches the listener in method parameters
           if (subscription && subscription.listener === listener) {
+            this._registry[registration].delete(subscription);
             subscription.remove();
             removed.push(registration);
             this.removeRegistration(registration);
@@ -278,7 +306,12 @@ class DatabaseSyncTree {
         subscription.remove();
       });
     } else {
-      SharedEventEmitter.addListener(eventRegistrationKey, listener);
+      const registration = SharedEventEmitter.addListener(eventRegistrationKey, listener);
+
+      // add this listener registration info to our emitter-key map
+      // in case we need to identify and remove a specific listener later
+      const registrations = this._allocate(this._registry, eventRegistrationKey);
+      registrations.add(registration);
     }
 
     return eventRegistrationKey;