Adding conversion support for JSON XHR requests.

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 '../../pkg/unittest/lib/unittest.dart';
-import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:async';
+import 'dart:convert';
 import 'dart:html';
-import "dart:convert";
 import 'dart:typed_data';
+import 'package:unittest/html_individual_config.dart';
+import 'package:unittest/unittest.dart';
 
 void fail(message) {
   guardAsync(() {
     expect(false, isTrue, reason: message);
   });
 }
 
 main() {
   useHtmlIndividualConfiguration();
   // Cache blocker is a workaround for:
@@ skipping 196 matching lines @@
           '${Uri.encodeQueryComponent(data[key])}');
       }
       var encodedData = parts.join('&');
 
       return HttpRequest.postFormData(
           '${window.location.protocol}//${window.location.host}/echo', data)
           .then((xhr) {
           expect(xhr.responseText, encodedData);
         });
     });
-
   });
 
   group('xhr_requestBlob', () {
     test('XHR.request responseType blob', () {
       if (Platform.supportsTypedData) {
         return HttpRequest.request(url, responseType: 'blob').then(
           (xhr) {
             expect(xhr.status, equals(200));
             var blob = xhr.response;
             expect(blob is Blob, isTrue);
             expect(blob, isNotNull);
           });
       }
     });
   });
+
+  group('json', () {
+    test('xhr responseType json', () {
+      var url = '${window.location.protocol}//${window.location.host}/echo';
+      var data = {
+        'key': 'value',
+        'a': 'b',
+        'one': 2,
+      };
+
+      HttpRequest.request(url,
+          method: 'POST',
+          sendData: JSON.encode(data),
+          responseType: 'json').then(
+          expectAsync1((xhr) {
+            expect(xhr.status, equals(200));
+            var json = xhr.response;
+            expect(json, equals(data));
+          }));
+    });
+  });
 }