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/public_api.ts

@@ -0,0 +1,18 @@
+/**
+ * @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
+ */
+
+export {HttpBackend, HttpHandler} from './src/backend';
+export {HttpClient} from './src/client';
+export {HttpHeaders} from './src/headers';
+export {HTTP_INTERCEPTORS, HttpInterceptor} from './src/interceptor';
+export {JsonpClientBackend, JsonpInterceptor} from './src/jsonp';
+export {HttpClientJsonpModule, HttpClientModule, interceptingHandler as ɵinterceptingHandler} from './src/module';
+export {HttpRequest} from './src/request';
+export {HttpDownloadProgressEvent, HttpErrorResponse, HttpEvent, HttpEventType, HttpHeaderResponse, HttpProgressEvent, HttpResponse, HttpResponseBase, HttpSentEvent, HttpUserEvent} from './src/response';
+export {HttpStandardUrlParameterCodec, HttpUrlEncodedBody, HttpUrlParameterCodec} from './src/url_encoded_body';
+export {HttpXhrBackend, XhrFactory} from './src/xhr';