feat(service-worker): introduce the @angular/service-worker package (#19274)

This service worker is a conceptual derivative of the existing @angular/service-worker maintained at github.com/angular/mobile-toolkit, but has been rewritten to support use across a much wider variety of applications.

Entrypoints include:

@angular/service-worker: a library for use within Angular client apps to communicate with the service worker.
@angular/service-worker/gen: a library for generating ngsw.json files from glob-based SW config files.
@angular/service-worker/ngsw-worker.js: the bundled service worker script itself.
@angular/service-worker/ngsw-cli.js: a CLI tool for generating ngsw.json files from glob-based SW config files.

packages/service-worker/src/update.ts

@@ -0,0 +1,42 @@
+/**
+ * @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
+ */
+
+import {Injectable} from '@angular/core';
+import {Observable} from 'rxjs/Observable';
+import {defer as obs_defer} from 'rxjs/observable/defer';
+import {map as op_map} from 'rxjs/operator/map';
+
+import {NgswCommChannel, UpdateActivatedEvent, UpdateAvailableEvent} from './low_level';
+
+
+/**
+ * Subscribe to update notifications from the Service Worker, trigger update
+ * checks, and forcibly activate updates.
+ *
+ * @experimental
+ */
+@Injectable()
+export class SwUpdate {
+  readonly available: Observable<UpdateAvailableEvent>;
+  readonly activated: Observable<UpdateActivatedEvent>;
+
+  constructor(private sw: NgswCommChannel) {
+    this.available = this.sw.eventsOfType('UPDATE_AVAILABLE');
+    this.activated = this.sw.eventsOfType('UPDATE_ACTIVATED');
+  }
+
+  checkForUpdate(): Promise<void> {
+    const statusNonce = this.sw.generateNonce();
+    return this.sw.postMessageWithStatus('CHECK_FOR_UPDATES', {statusNonce}, statusNonce);
+  }
+
+  activateUpdate(): Promise<void> {
+    const statusNonce = this.sw.generateNonce();
+    return this.sw.postMessageWithStatus('ACTIVATE_UPDATE', {statusNonce}, statusNonce);
+  }
+}