Remove usage of dart:json.

pkg/oauth2/lib/src/credentials.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 credentials;
 
 import 'dart:async';
-import 'dart:json' as JSON;
+import 'dart:convert';
 
 import 'package:http/http.dart' as http;
 
 import 'handle_access_token_response.dart';
 import 'utils.dart';
 
 /// Credentials that prove that a client is allowed to access a resource on the
 /// resource owner's behalf. These credentials are long-lasting and can be
 /// safely persisted across multiple runs of the program.
 ///
@@ skipping 54 matching lines @@
   /// [FormatException] if the JSON is incorrectly formatted.
   factory Credentials.fromJson(String json) {
     void validate(bool condition, String message) {
       if (condition) return;
       throw new FormatException(
           "Failed to load credentials: $message.\n\n$json");
     }
 
     var parsed;
     try {
-      parsed = JSON.parse(json);
+      parsed = JSON.decode(json);
     } on FormatException catch (e) {
       validate(false, 'invalid JSON');
     }
 
     validate(parsed is Map, 'was not a JSON map');
     validate(parsed.containsKey('accessToken'),
         'did not contain required field "accessToken"');
     validate(parsed['accessToken'] is String,
         'required field "accessToken" was not a string, was '
         '${parsed["accessToken"]}');
@@ skipping 24 matching lines @@
         parsed['accessToken'],
         parsed['refreshToken'],
         tokenEndpoint,
         scopes,
         expiration);
   }
 
   /// Serializes a set of credentials to JSON. Nothing is guaranteed about the
   /// output except that it's valid JSON and compatible with
   /// [Credentials.toJson].
-  String toJson() => JSON.stringify({
+  String toJson() => JSON.encode({
     'accessToken': accessToken,
     'refreshToken': refreshToken,
     'tokenEndpoint': tokenEndpoint == null ? null : tokenEndpoint.toString(),
     'scopes': scopes,
     'expiration': expiration == null ? null : expiration.millisecondsSinceEpoch
   });
 
   /// Returns a new set of refreshed credentials. See [Client.identifier] and
   /// [Client.secret] for explanations of those parameters.
   ///
@@ skipping 44 matching lines @@
       if (credentials.refreshToken != null) return credentials;
       return new Credentials(
           credentials.accessToken,
           this.refreshToken,
           credentials.tokenEndpoint,
           credentials.scopes,
           credentials.expiration);
     });
   }
 }