Remove usage of dart:json.

samples/chat/dart_client/chat.dart

Index: samples/chat/dart_client/chat.dart
diff --git a/samples/chat/dart_client/chat.dart b/samples/chat/dart_client/chat.dart
index a0a154f3f6d99de554e2aab5cbcac742d6906308..a687a1342d7525c0b049aa10bc36400a86064900 100644
--- a/samples/chat/dart_client/chat.dart
+++ b/samples/chat/dart_client/chat.dart
@@ -4,7 +4,7 @@
 
 library chat;
 import 'dart:html';
-import 'dart:json' as jsonlib;
+import "dart:convert";
 
 void main() {
   new Chat().start();
@@ -163,14 +163,14 @@ class Chat {
     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;
   }