feat(http.ts): export BrowserXHR

needed for replacing BrowserXHR bindings with mock/server version  etc

Closes angular#2641

Author: PatrickJS

modules/angular2/http.ts

@@ -12,11 +12,11 @@ import {BrowserXhr} from 'angular2/src/http/backends/browser_xhr';
 import {BrowserJsonp} from 'angular2/src/http/backends/browser_jsonp';
 import {BaseRequestOptions, RequestOptions} from 'angular2/src/http/base_request_options';
 import {ConnectionBackend} from 'angular2/src/http/interfaces';
+import {BaseResponseOptions, ResponseOptions} from 'angular2/src/http/base_response_options';
 
 export {MockConnection, MockBackend} from 'angular2/src/http/backends/mock_backend';
 export {Request} from 'angular2/src/http/static_request';
 export {Response} from 'angular2/src/http/static_response';
-import {BaseResponseOptions, ResponseOptions} from 'angular2/src/http/base_response_options';
 
 export {
   IRequestOptions,
@@ -25,6 +25,7 @@ export {
   ConnectionBackend
 } from 'angular2/src/http/interfaces';
 
+export {BrowserXhr} from 'angular2/src/http/backends/browser_xhr';
 export {BaseRequestOptions, RequestOptions} from 'angular2/src/http/base_request_options';
 export {BaseResponseOptions, ResponseOptions} from 'angular2/src/http/base_response_options';
 export {XHRBackend, XHRConnection} from 'angular2/src/http/backends/xhr_backend';