test(service-worker): improve adding clients in `SwTestHarness` (#23625)

This commits changes how clients are added in `SwTestHarness`, so that
the behavior in tests closer mimics what would happen in an actual
ServiceWorker.
It also removes auto-adding clients when calling `clients.get()`, which
could hide bugs related to non-existing clients.

PR Close #23625

packages/service-worker/worker/test/happy_spec.ts

@@ -483,7 +483,6 @@ const manifestUpdateHash = sha1(JSON.stringify(manifestUpdate));
     async_it('shows notifications for push notifications', async() => {
       expect(await makeRequest(scope, '/foo.txt')).toEqual('this is foo');
       await driver.initialized;
-      scope.clients.add('default');
       await scope.handlePush({
         notification: {
           title: 'This is a test',
@@ -665,7 +664,7 @@ const manifestUpdateHash = sha1(JSON.stringify(manifestUpdate));
         server.assertSawRequestFor('/baz');
       });
 
-      async_it('does not redirect to index on a request that does not expect HTML', async() => {
+      async_it('does not redirect to index on a request that does not accept HTML', async() => {
         expect(await navRequest('/baz', {headers: {}})).toBeNull();
         server.assertSawRequestFor('/baz');
 
@@ -793,7 +792,6 @@ async function makeRequest(
   const [resPromise, done] = scope.handleFetch(new MockRequest(url, init), clientId);
   await done;
   const res = await resPromise;
-  scope.clients.add(clientId);
   if (res !== undefined && res.ok) {
     return res.text();
   }

packages/service-worker/worker/testing/scope.ts

@@ -62,10 +62,7 @@ export class MockClients implements Clients {
 
   remove(clientId: string): void { this.clients.delete(clientId); }
 
-  async get(id: string): Promise<Client> {
-    this.add(id);
-    return this.clients.get(id) !as any as Client;
-  }
+  async get(id: string): Promise<Client> { return this.clients.get(id) !as any as Client; }
 
   getMock(id: string): MockClient|undefined { return this.clients.get(id); }
 
@@ -197,6 +194,10 @@ export class SwTestHarness implements ServiceWorkerGlobalScope, Adapter, Context
     const event = new MockFetchEvent(req, clientId || null);
     this.eventHandlers.get('fetch') !.call(this, event);
 
+    if (clientId) {
+      this.clients.add(clientId);
+    }
+
     return [event.response, event.ready];
   }