fix(XHRImpl): fix errors, add a spec

fixes #1715

modules/angular2/src/services/xhr_impl.dart

@@ -1,15 +1,17 @@
 library angular2.src.services.xhr_impl;
 
-import 'dart:async';
-import 'dart:html';
+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 request) => request.responseText,
-        onError: (Error e) => throw 'Failed to load $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

@@ -15,11 +15,11 @@ export class XHRImpl extends XHR {
       if (200 <= status && status <= 300) {
         completer.resolve(xhr.responseText);
       } else {
-        completer.reject(`Failed to load ${url}`);
+        completer.reject(`Failed to load ${url}`, null);
       }
     };
 
-    xhr.onerror = function() { completer.reject(`Failed to load ${url}`); };
+    xhr.onerror = function() { completer.reject(`Failed to load ${url}`, null); };
 
     xhr.send();
     return completer.promise;