fix(app): fix issue where instanceIdentifier wasn't being passed

Fixed an issue where instance identifiers weren't being passed to their creation factories. Added
some tests to validate that we don't accidentally remove that in the future.

src/app/firebase_app.ts

@@ -303,7 +303,7 @@ class FirebaseAppImpl implements FirebaseApp {
     }
 
     if (!this.services_[name][instanceIdentifier]) {
-      let service = this.firebase_.INTERNAL.factories[name](this, this.extendApp.bind(this));
+      let service = this.firebase_.INTERNAL.factories[name](this, this.extendApp.bind(this), instanceIdentifier);
       this.services_[name][instanceIdentifier] = service;
     }
 

tests/app/unit/firebase_app.test.ts

@@ -232,7 +232,10 @@ describe("Firebase App Class", () => {
     // Register Multi Instance Service
     firebase.INTERNAL.registerService(
       'multiInstance',
-      app => new TestService(app),
+      (...args) => {
+        const [app,,instanceIdentifier] = args;
+        return new TestService(app, instanceIdentifier);
+      },
       null,
       null,
       true
@@ -249,6 +252,7 @@ describe("Firebase App Class", () => {
     assert.strictEqual(service2, (firebase.app() as any).multiInstance(serviceIdentifier));
 
     // Ensure that the two services **are not equal**
+    assert.notStrictEqual(service.instanceIdentifier, service2.instanceIdentifier);
     assert.notStrictEqual(service, service2);
     assert.notStrictEqual((firebase.app() as any).multiInstance(), (firebase.app() as any).multiInstance(serviceIdentifier));
   });
@@ -257,7 +261,10 @@ describe("Firebase App Class", () => {
     // Register Multi Instance Service
     firebase.INTERNAL.registerService(
       'singleInstance',
-      app => new TestService(app),
+      (...args) => {
+        const [app,,instanceIdentifier] = args;
+        return new TestService(app, instanceIdentifier)
+      },
       null,
       null,
       false // <-- multi instance flag
@@ -286,9 +293,7 @@ describe("Firebase App Class", () => {
 });
 
 class TestService implements FirebaseService {
-  constructor(private app_: FirebaseApp) {
-    // empty
-  }
+  constructor(private app_: FirebaseApp, public instanceIdentifier?: string) {}
 
   // TODO(koss): Shouldn't this just be an added method on
   // the service instance?