Remove usage of dart:json.

pkg/oauth2/test/client_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 client_test;
 
 import 'dart:async';
-import 'dart:json' as JSON;
+import 'dart:convert';
 
 import 'package:http/http.dart' as http;
 import 'package:oauth2/oauth2.dart' as oauth2;
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 final Uri requestUri = Uri.parse("http://example.com/resource");
 
 final Uri tokenEndpoint = Uri.parse('http://example.com/token');
@@ skipping 23 matching lines @@
         "request", () {
       var expiration = new DateTime.now().subtract(new Duration(hours: 1));
       var credentials = new oauth2.Credentials(
           'access token', 'refresh token', tokenEndpoint, [], expiration);
       var client = new oauth2.Client('identifier', 'secret', credentials,
           httpClient: httpClient);
 
       httpClient.expectRequest((request) {
         expect(request.method, equals('POST'));
         expect(request.url.toString(), equals(tokenEndpoint.toString()));
-        return new Future.value(new http.Response(JSON.stringify({
+        return new Future.value(new http.Response(JSON.encode({
           'access_token': 'new access token',
           'token_type': 'bearer'
         }), 200, headers: {'content-type': 'application/json'}));
       });
 
       httpClient.expectRequest((request) {
         expect(request.method, equals('GET'));
         expect(request.url.toString(), equals(requestUri.toString()));
         expect(request.headers['authorization'],
             equals('Bearer new access token'));
@@ skipping 29 matching lines @@
 
     test("can manually refresh the credentials", () {
       var credentials = new oauth2.Credentials(
           'access token', 'refresh token', tokenEndpoint);
       var client = new oauth2.Client('identifier', 'secret', credentials,
           httpClient: httpClient);
 
       httpClient.expectRequest((request) {
         expect(request.method, equals('POST'));
         expect(request.url.toString(), equals(tokenEndpoint.toString()));
-        return new Future.value(new http.Response(JSON.stringify({
+        return new Future.value(new http.Response(JSON.encode({
           'access_token': 'new access token',
           'token_type': 'bearer'
         }), 200, headers: {'content-type': 'application/json'}));
       });
 
       expect(client.refreshCredentials().then((_) {
         expect(client.credentials.accessToken, equals('new access token'));
       }), completes);
     });
 
@@ skipping 105 matching lines @@
         return new Future.value(new http.Response('bad job', 401,
                 headers: {'www-authenticate': 'Bearer'}));
       });
 
       expect(
           client.get(requestUri).then((response) => response.statusCode),
           completion(equals(401)));
     });
   });
 }