Remove usage of dart:json.

samples/chat/dart_client/chat.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 chat;
 import 'dart:html';
-import 'dart:json' as jsonlib;
+import "dart:convert";
 
 void main() {
   new Chat().start();
 }
 
 class Chat {
   void start() {
     Document doc = window.document;
     _joinButton = doc.query("#joinButton");
     _leaveButton = doc.query("#leaveButton");
@@ skipping 138 matching lines @@
   void onPollFailed() {
     showStatus("Failed to receive messages. Please try again later.");
     uiJoin();
   }
 
   HttpRequest sendRequest(String url, Map json, var onSuccess, var onError) {
     HttpRequest request = new HttpRequest();
     request.onReadyStateChange.listen((Event event) {
       if (request.readyState != 4) return;
       if (request.status == 200) {
-        onSuccess(jsonlib.parse(request.responseText));
+        onSuccess(JSON.decode(request.responseText));
       } else {
         onError();
       }
     });
     request.open("POST", url, async: true);
     request.setRequestHeader("Content-Type", "text/plain;charset=UTF-8");
-    request.send(jsonlib.stringify(json));
+    request.send(JSON.encode(json));
     return request;
   }
 
   void uiJoin() {
     enableButton(_joinButton);
     showElement(_joinSection);
     hideElement(_chatSection);
     _nextMessage = 0;
     _messages.children.clear();
     showStatus("Welcome to dart chat sample. "
@@ skipping 122 matching lines @@
   Element _chatSection;
   InputElement _messageInput;
   Element _messages;
   Element _statusText;
 
   String _session = null;
   int _nextMessage = 0;
   HttpRequest _pollRequest = null;
 
 }