refactor(xhr): move render's xhr implementation to render/

The existence of this module in the services/ folder led some to believe xhr
is meant to be a general-purpose http library.

Fixes #2305

modules/angular2/src/services/xhr.ts

@@ -1,5 +0,0 @@
-import {Promise} from 'angular2/src/facade/async';
-
-export class XHR {
-  get(url: string): Promise<string> { return null; }
-}

modules/angular2/src/services/xhr_impl.dart

@@ -1,14 +0,0 @@
-library angular2.src.services.xhr_impl;
-
-import 'dart:async' show Future;
-import 'dart:html' show HttpRequest;
-import 'package:angular2/di.dart';
-import './xhr.dart' show XHR;
-
-@Injectable()
-class XHRImpl extends XHR {
-  Future<String> get(String url) {
-    return HttpRequest.request(url).then((HttpRequest req) => req.responseText,
-        onError: (_) => new Future.error('Failed to load $url'));
-  }
-}

modules/angular2/src/services/xhr_impl.ts

@@ -1,27 +0,0 @@
-import {Injectable} from 'angular2/di';
-import {Promise, PromiseWrapper} from 'angular2/src/facade/async';
-import {XHR} from './xhr';
-
-@Injectable()
-export class XHRImpl extends XHR {
-  get(url: string): Promise<string> {
-    var completer = PromiseWrapper.completer();
-    var xhr = new XMLHttpRequest();
-    xhr.open('GET', url, true);
-    xhr.responseType = 'text';
-
-    xhr.onload = function() {
-      var status = xhr.status;
-      if (200 <= status && status <= 300) {
-        completer.resolve(xhr.responseText);
-      } else {
-        completer.reject(`Failed to load ${url}`, null);
-      }
-    };
-
-    xhr.onerror = function() { completer.reject(`Failed to load ${url}`, null); };
-
-    xhr.send();
-    return completer.promise;
-  }
-}