Remove usage of dart:json.

pkg/oauth2/test/authorization_code_grant_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 authorization_code_grant_test;
 
 import 'dart:async';
-import 'dart:json' as JSON;
+import 'dart:convert';
 
 import 'package:unittest/unittest.dart';
 import 'package:http/http.dart' as http;
 import 'package:oauth2/oauth2.dart' as oauth2;
 
 import 'utils.dart';
 
 final redirectUrl = Uri.parse('http://example.com/redirect');
 
 ExpectClient client;
@@ skipping 125 matching lines @@
         expect(request.method, equals('POST'));
         expect(request.url.toString(), equals(grant.tokenEndpoint.toString()));
         expect(request.bodyFields, equals({
           'grant_type': 'authorization_code',
           'code': 'auth code',
           'redirect_uri': redirectUrl.toString(),
           'client_id': 'identifier',
           'client_secret': 'secret'
         }));
 
-        return new Future.value(new http.Response(JSON.stringify({
+        return new Future.value(new http.Response(JSON.encode({
           'access_token': 'access token',
           'token_type': 'bearer',
         }), 200, headers: {'content-type': 'application/json'}));
       });
 
       grant.handleAuthorizationResponse({'code': 'auth code'})
           .then(expectAsync1((client) {
             expect(client.credentials.accessToken, equals('access token'));
           }));
     });
@@ skipping 23 matching lines @@
         expect(request.method, equals('POST'));
         expect(request.url.toString(), equals(grant.tokenEndpoint.toString()));
         expect(request.bodyFields, equals({
           'grant_type': 'authorization_code',
           'code': 'auth code',
           'redirect_uri': redirectUrl.toString(),
           'client_id': 'identifier',
           'client_secret': 'secret'
         }));
 
-        return new Future.value(new http.Response(JSON.stringify({
+        return new Future.value(new http.Response(JSON.encode({
           'access_token': 'access token',
           'token_type': 'bearer',
         }), 200, headers: {'content-type': 'application/json'}));
       });
 
       expect(grant.handleAuthorizationCode('auth code'),
           completion(predicate((client) {
             expect(client.credentials.accessToken, equals('access token'));
             return true;
           })));
     });
   });
 }