fix(common): temporarily re-export and deprecate XhrFactory (#41393)

The moved `XhrFactory` still needs to be available from `@angular/common/http`
for some libraries that were built prior to 12.0.0, otherwise they cannot be
used in applications built post-12.0.0.

This commit adds back the re-export of `XhrFactory` and deprecates it.

PR Close #41393

aio/content/guide/deprecations.md

@@ -40,6 +40,7 @@ v12 - v15
 | ----------------------------- | ---------------------------------------------------------------------------   | ----------------- |
 | `@angular/common`             | [`ReflectiveInjector`](#reflectiveinjector)                                   | <!--v8--> v11 |
 | `@angular/common`             | [`CurrencyPipe` - `DEFAULT_CURRENCY_CODE`](api/common/CurrencyPipe#currency-code-deprecation) | <!--v9--> v11 |
+| `@angular/common/http`        | [`XhrFactory`](api/common/http/XhrFactory)                                    | <!--v12--> v15 |
 | `@angular/core`               | [`DefaultIterableDiffer`](#core)                                              | <!--v7--> v11 |
 | `@angular/core`               | [`ReflectiveKey`](#core)                                                      | <!--v8--> v11 |
 | `@angular/core`               | [`RenderComponentType`](#core)                                                | <!--v7--> v11 |
@@ -82,6 +83,14 @@ Tip: In the [API reference section](api) of this doc site, deprecated APIs are i
 | [`CurrencyPipe` - `DEFAULT_CURRENCY_CODE`](api/common/CurrencyPipe#currency-code-deprecation) | `{provide: DEFAULT_CURRENCY_CODE, useValue: 'USD'}` | v9                    | From v11 the default code will be extracted from the locale data given by `LOCAL_ID`, rather than `USD`. |
 
 
+{@a common-http}
+### @angular/common/http
+
+| API                                          | Replacement                          | Deprecation announced | Notes |
+| -------------------------------------------- | ------------------------------------ | --------------------- | ----- |
+| [`XhrFactory`](api/common/http/XhrFactory)   | `XhrFactory` in `@angular/common`    | v12                   | The `XhrFactory` has moved from `@angular/common/http` to `@angular/common`. |
+
+
 {@a core}
 ### @angular/core
 

goldens/public-api/common/http/http.d.ts

@@ -1930,7 +1930,7 @@ export declare interface HttpUserEvent<T> {
 }
 
 export declare class HttpXhrBackend implements HttpBackend {
-    constructor(xhrFactory: XhrFactory);
+    constructor(xhrFactory: XhrFactory_2);
     handle(req: HttpRequest<any>): Observable<HttpEvent<any>>;
 }
 
@@ -1947,3 +1947,6 @@ export declare class JsonpInterceptor {
     constructor(jsonp: JsonpClientBackend);
     intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>>;
 }
+
+/** @deprecated */
+export declare type XhrFactory = XhrFactory_2;

packages/common/http/public_api.ts

@@ -6,6 +6,27 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import {XhrFactory as XhrFactory_fromAngularCommon} from '@angular/common';
+
+/**
+ * A wrapper around the `XMLHttpRequest` constructor.
+ *
+ * @publicApi
+ * @see `XhrFactory`
+ * @deprecated
+ * `XhrFactory` has moved, please import `XhrFactory` from `@angular/common` instead.
+ */
+export type XhrFactory = XhrFactory_fromAngularCommon;
+/**
+ * A wrapper around the `XMLHttpRequest` constructor.
+ *
+ * @publicApi
+ * @see `XhrFactory`
+ * @deprecated
+ * `XhrFactory` has moved, please import `XhrFactory` from `@angular/common` instead.
+ */
+export const XhrFactory = XhrFactory_fromAngularCommon;
+
 export {HttpBackend, HttpHandler} from './src/backend';
 export {HttpClient} from './src/client';
 export {HttpContext, HttpContextToken} from './src/context';