fix(common): do not override locale provided on bootstrap (#13654)

Closes #13607

modules/@angular/core/src/application_module.ts

@@ -11,6 +11,7 @@ import {ApplicationInitStatus} from './application_init';
 import {ApplicationRef, ApplicationRef_} from './application_ref';
 import {APP_ID_RANDOM_PROVIDER} from './application_tokens';
 import {IterableDiffers, KeyValueDiffers, defaultIterableDiffers, defaultKeyValueDiffers} from './change_detection/change_detection';
+import {Inject, Optional, SkipSelf} from './di/metadata';
 import {LOCALE_ID} from './i18n/tokens';
 import {Compiler} from './linker/compiler';
 import {ViewUtils} from './linker/view_utils';
@@ -24,6 +25,10 @@ export function _keyValueDiffersFactory() {
   return defaultKeyValueDiffers;
 }
 
+export function _localeFactory(locale?: string): string {
+  return locale || 'en-US';
+}
+
 /**
  * This module includes the providers of @angular/core that are needed
  * to bootstrap components via `ApplicationRef`.
@@ -41,7 +46,11 @@ export function _keyValueDiffersFactory() {
     AnimationQueue,
     {provide: IterableDiffers, useFactory: _iterableDiffersFactory},
     {provide: KeyValueDiffers, useFactory: _keyValueDiffersFactory},
-    {provide: LOCALE_ID, useValue: 'en-US'},
+    {
+      provide: LOCALE_ID,
+      useFactory: _localeFactory,
+      deps: [[new Inject(LOCALE_ID), new Optional(), new SkipSelf()]]
+    },
   ]
 })
 export class ApplicationModule {

modules/@angular/core/test/application_module_spec.ts

@@ -14,4 +14,4 @@ export function main() {
     it('should set the default locale to "en-US"',
        inject([LOCALE_ID], (defaultLocale: string) => { expect(defaultLocale).toEqual('en-US'); }));
   });
-}
+}