Define _FirebaseService interface (firebase#3112)

Author: Feiyang1

common/api-review/app-types-exp.api.md

@@ -53,6 +53,13 @@ export interface FirebaseOptions {
   storageBucket?: string;
 }
 
+// @internal (undocumented)
+export interface _FirebaseService {
+  // (undocumented)
+  app: FirebaseApp;
+   delete(): Promise<void>;
+}
+
 // @public (undocumented)
 export interface PlatformLoggerService {
   // (undocumented)

packages-exp/app-types-exp/index.d.ts

@@ -104,6 +104,18 @@ export interface PlatformLoggerService {
   getPlatformInfoString(): string;
 }
 
+/**
+ * @internal
+ */
+export interface _FirebaseService {
+  app: FirebaseApp;
+  /**
+   * Delete the service and free it's resources - called from
+   * {@link @firebase/app-exp#deleteApp | deleteApp()}
+   */
+  delete(): Promise<void>;
+}
+
 declare module '@firebase/component' {
   interface NameServiceMapping {
     'app-exp': FirebaseApp;