feat: deprecate @angular/http in favor of @angular/common/http (#18906)

As of 5.0, @angular/http is deprecated. @angular/common/http will
be the official HTTP API in Angular going forward.

PR Close #18906

packages/common/http/testing/src/backend.ts

@@ -25,7 +25,7 @@ import {TestRequest} from './request';
  * requests were made and then flush them. In the end, a verify() method asserts
  * that no unexpected requests were made.
  *
- * @experimental
+ * @stable
  */
 @Injectable()
 export class HttpClientTestingBackend implements HttpBackend, HttpTestingController {