Use a mock back end for HTTP calls

- [Instructor] To work with HTTP calls from Angular…you typically need some sort of end point to call to,…but since Angular has a powerful dependency injection engine…and the code base is built upon it…we can actually leverage that to replace Angular's…HTTP XHR Backend class…that handles the HTTP calls…and set up a mock one to simulate those calls.…The exercise files have already been set up…with a mock class for that.…If you are interested in it,…feel free to check out the mock-xhr-backend.ts file…to see what it's up to.…

This class is set up in the app…to support HTTP calls to the URL, media items,…forget posts, and delete verbs.…It will allow us to build out the media item service…to use Angular's HTTP service…while not requiring a separate API to hit.…And with Angular's DI architecture…the default HTTP XHR Backend class…is easy to replace.…Over in the app.module.ts file…we need to import the type that we are going to mock.…

The HTTP XHR Backend.…That comes from @angular/common/http.…So, we can add HttpXhrBackend to the imports list.…