feat(common): new HttpClient API

HttpClient is an evolution of the existing Angular HTTP API, which exists
alongside of it in a separate package, @angular/common/http. This structure
ensures that existing codebases can slowly migrate to the new API.

The new API improves significantly on the ergonomics and features of the legacy
API. A partial list of new features includes:

* Typed, synchronous response body access, including support for JSON body types
* JSON is an assumed default and no longer needs to be explicitly parsed
* Interceptors allow middleware logic to be inserted into the pipeline
* Immutable request/response objects
* Progress events for both request upload and response download
* Post-request verification & flush based testing framework

packages/common/http/src/backend.ts

@@ -0,0 +1,25 @@
+/**
+ * @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 {Observable} from 'rxjs/Observable';
+import {HttpRequest} from './request';
+import {HttpEvent} from './response';
+
+/**
+ * @experimental
+ */
+export abstract class HttpHandler {
+  abstract handle(req: HttpRequest<any>): Observable<HttpEvent<any>>;
+}
+
+/**
+ * @experimental
+ */
+export abstract class HttpBackend implements HttpHandler {
+  abstract handle(req: HttpRequest<any>): Observable<HttpEvent<any>>;
+}