fix(app, types): initializeApp returns Promise<FirebaseApp>

previously was <FirebaseApp> so it seemed sync vs async

All of the E2E tests in the area have been enabled and tested
(by purposefully modifying expected output to not match) to fail
as expected, and pass with correct expected output

packages/app/e2e/app.e2e.js

@@ -37,24 +37,10 @@ describe('firebase', function () {
     should.equal(firebase.app().options.storageBucket, platformAppConfig.storageBucket);
   });
 
-  xit('it should initialize dynamic apps', function () {
-    const name = `testscoreapp${FirebaseHelpers.id}`;
-    const platformAppConfig = FirebaseHelpers.app.config();
-    return firebase.initializeApp(platformAppConfig, name).then(newApp => {
-      newApp.name.should.equal(name);
-      newApp.toString().should.equal(name);
-      newApp.options.apiKey.should.equal(platformAppConfig.apiKey);
-      return newApp.delete();
-    });
-  });
-
   it('SDK_VERSION should return a string version', function () {
     firebase.SDK_VERSION.should.be.a.String();
   });
-});
 
-// eslint-disable-next-line mocha/max-top-level-suites
-describe('firebase -> X', function () {
   it('apps should provide an array of apps', function () {
     should.equal(!!firebase.apps.length, true);
     should.equal(firebase.apps.includes(firebase.app('[DEFAULT]')), true);
@@ -66,31 +52,55 @@ describe('firebase -> X', function () {
     should.equal(firebase.app().automaticDataCollectionEnabled, false);
   });
 
-  xit('apps can be deleted', async function () {
+  it('it should initialize dynamic apps', async function () {
     const name = `testscoreapp${FirebaseHelpers.id}`;
     const platformAppConfig = FirebaseHelpers.app.config();
     const newApp = await firebase.initializeApp(platformAppConfig, name);
-
     newApp.name.should.equal(name);
     newApp.toString().should.equal(name);
     newApp.options.apiKey.should.equal(platformAppConfig.apiKey);
+    return newApp.delete();
+  });
 
-    await newApp.delete();
+  it('should error if dynamic app initialization values are incorrect', async function () {
+    try {
+      await firebase.initializeApp({ appId: 'myid' }, 'myname');
+      throw new Error('Should have rejected incorrect initializeApp input');
+    } catch (e) {
+      e.message.should.equal("Missing or invalid FirebaseOptions property 'apiKey'.");
+    }
+  });
 
-    (() => {
-      newApp.delete();
-    }).should.throw(`Firebase App named '${name}' already deleted`);
+  it('apps can be deleted, but only once', async function () {
+    const name = `testscoreapp${FirebaseHelpers.id}`;
+    const platformAppConfig = FirebaseHelpers.app.config();
+    const newApp = await firebase.initializeApp(platformAppConfig, name);
 
-    (() => {
+    newApp.name.should.equal(name);
+    newApp.toString().should.equal(name);
+    newApp.options.apiKey.should.equal(platformAppConfig.apiKey);
+
+    await newApp.delete();
+    try {
+      await newApp.delete();
+    } catch (e) {
+      e.message.should.equal(`Firebase App named '${name}' already deleted`);
+    }
+    try {
       firebase.app(name);
-    }).should.throw(`No Firebase App '${name}' has been created - call firebase.initializeApp()`);
+    } catch (e) {
+      e.message.should.equal(
+        `No Firebase App '${name}' has been created - call firebase.initializeApp()`,
+      );
+    }
   });
 
-  xit('prevents the default app from being deleted', async function () {
-    firebase
-      .app()
-      .delete()
-      .should.be.rejectedWith('Unable to delete the default native firebase app instance.');
+  it('prevents the default app from being deleted', async function () {
+    try {
+      await firebase.app().delete();
+    } catch (e) {
+      e.message.should.equal('Unable to delete the default native firebase app instance.');
+    }
   });
 
   it('extendApp should provide additional functionality', function () {

packages/app/lib/index.d.ts

@@ -159,7 +159,7 @@ export namespace ReactNativeFirebase {
      * @param options Options to configure the services used in the App.
      * @param config The optional config for your firebase app
      */
-    initializeApp(options: FirebaseAppOptions, config?: FirebaseAppConfig): FirebaseApp;
+    initializeApp(options: FirebaseAppOptions, config?: FirebaseAppConfig): Promise<FirebaseApp>;
 
     /**
      * Create (and initialize) a FirebaseApp.
@@ -168,7 +168,7 @@ export namespace ReactNativeFirebase {
      * @param name The optional name of the app to initialize ('[DEFAULT]' if
      * omitted)
      */
-    initializeApp(options: FirebaseAppOptions, name?: string): FirebaseApp;
+    initializeApp(options: FirebaseAppOptions, name?: string): Promise<FirebaseApp>;
 
     /**
      * Retrieve an instance of a FirebaseApp.