refactor: export DEFAULT_DATABASE_ID from src/firestore/index.ts

Address review feedback: instead of duplicating the '(default)' string
in 3 places, export the constant from the source module and import it
in test files.

Author: ouzkilic

src/firestore/index.ts

@@ -24,7 +24,7 @@ import { Firestore } from '@google-cloud/firestore';
 import { App, getApp } from '../app';
 import { FirebaseApp } from '../app/firebase-app';
 import { FirestoreService, FirestoreSettings } from './firestore-internal';
-const DEFAULT_DATABASE_ID = '(default)';
+export const DEFAULT_DATABASE_ID = '(default)';
 
 export {
   AddPrefixToKeys,

test/unit/firestore/firestore.spec.ts

@@ -28,7 +28,7 @@ import {
 } from '../../../src/app/credential-internal';
 import { FirestoreService, getFirestoreOptions } from '../../../src/firestore/firestore-internal';
 import { getSdkVersion } from '../../../src/utils/index';
-const DEFAULT_DATABASE_ID = '(default)';
+import { DEFAULT_DATABASE_ID } from '../../../src/firestore/index';
 
 describe('Firestore', () => {
   let mockApp: FirebaseApp;

test/unit/firestore/index.spec.ts

@@ -23,8 +23,7 @@ import * as chaiAsPromised from 'chai-as-promised';
 
 import * as mocks from '../../resources/mocks';
 import { App } from '../../../src/app/index';
-import { getFirestore, initializeFirestore, Firestore } from '../../../src/firestore/index';
-const DEFAULT_DATABASE_ID = '(default)';
+import { getFirestore, initializeFirestore, Firestore, DEFAULT_DATABASE_ID } from '../../../src/firestore/index';
 
 chai.should();
 chai.use(sinonChai);