feat: add a PLATFORM_ID token that provides a platform id Object. (#14647)

modules/@angular/common/src/common.ts

@@ -16,4 +16,5 @@ export {NgLocaleLocalization, NgLocalization} from './localization';
 export {CommonModule, DeprecatedCommonModule} from './common_module';
 export {NgClass, NgFor, NgForOf, NgIf, NgPlural, NgPluralCase, NgStyle, NgSwitch, NgSwitchCase, NgSwitchDefault, NgTemplateOutlet, NgComponentOutlet} from './directives/index';
 export {AsyncPipe, DatePipe, I18nPluralPipe, I18nSelectPipe, JsonPipe, LowerCasePipe, CurrencyPipe, DecimalPipe, PercentPipe, SlicePipe, UpperCasePipe, TitleCasePipe} from './pipes/index';
+export {PLATFORM_BROWSER_ID as ɵPLATFORM_BROWSER_ID, PLATFORM_SERVER_ID as ɵPLATFORM_SERVER_ID, PLATFORM_WORKER_APP_ID as ɵPLATFORM_WORKER_APP_ID, PLATFORM_WORKER_UI_ID as ɵPLATFORM_WORKER_UI_ID, isPlatformBrowser, isPlatformServer, isPlatformWorkerApp, isPlatformWorkerUi} from './platform_id';
 export {VERSION} from './version';

modules/@angular/common/src/platform_id.ts

@@ -0,0 +1,44 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+export const PLATFORM_BROWSER_ID = 'browser';
+export const PLATFORM_SERVER_ID = 'server';
+export const PLATFORM_WORKER_APP_ID = 'browserWorkerApp';
+export const PLATFORM_WORKER_UI_ID = 'browserWorkerUi';
+
+/**
+ * Returns whether a platform id represents a browser platform.
+ * @experimental
+ */
+export function isPlatformBrowser(platformId: Object): boolean {
+  return platformId === PLATFORM_BROWSER_ID;
+}
+
+/**
+ * Returns whether a platform id represents a server platform.
+ * @experimental
+ */
+export function isPlatformServer(platformId: Object): boolean {
+  return platformId === PLATFORM_SERVER_ID;
+}
+
+/**
+ * Returns whether a platform id represents a web worker app platform.
+ * @experimental
+ */
+export function isPlatformWorkerApp(platformId: Object): boolean {
+  return platformId === PLATFORM_WORKER_APP_ID;
+}
+
+/**
+ * Returns whether a platform id represents a web worker UI platform.
+ * @experimental
+ */
+export function isPlatformWorkerUi(platformId: Object): boolean {
+  return platformId === PLATFORM_WORKER_UI_ID;
+}