Adding HttpRequest.responseHeaders

tests/html/xhr_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library XHRTest;
 import 'dart:async';
 import 'dart:convert';
 import 'dart:html';
 import 'dart:typed_data';
 import 'package:unittest/html_individual_config.dart';
@@ skipping 228 matching lines @@
           method: 'POST',
           sendData: JSON.encode(data),
           responseType: 'json').then(
           expectAsync1((xhr) {
             expect(xhr.status, equals(200));
             var json = xhr.response;
             expect(json, equals(data));
           }));
     });
   });
+
+  group('headers', () {
+    test('xhr responseHeaders', () {
+      return HttpRequest.request(url, mimeType: 'application/binary').then(
+        (xhr) {
+          expect(xhr.responseHeaders['content-type'], 'application/binary');
+        });
+    });
+  });
 }