Remove usage of dart:json.

sdk/lib/_internal/pub/test/oauth2/utils.dart

Index: sdk/lib/_internal/pub/test/oauth2/utils.dart
diff --git a/sdk/lib/_internal/pub/test/oauth2/utils.dart b/sdk/lib/_internal/pub/test/oauth2/utils.dart
index a67e2bc4c39ee1a1fe4424276242738ed9315080..019628b3fbddb534a3d3853ed9e98d698aba5bfc 100644
--- a/sdk/lib/_internal/pub/test/oauth2/utils.dart
+++ b/sdk/lib/_internal/pub/test/oauth2/utils.dart
@@ -4,8 +4,8 @@
 
 library oauth2.utils;
 
+import 'dart:convert';
 import 'dart:io';
-import 'dart:json' as json;
 
 import 'package:http/http.dart' as http;
 import 'package:scheduled_test/scheduled_process.dart';
@@ -45,7 +45,7 @@ void handleAccessTokenRequest(ScheduledServer server, String accessToken) {
 
       request.response.headers.contentType =
           new ContentType("application", "json");
-      request.response.write(json.stringify({
+      request.response.write(JSON.encode({
         "access_token": accessToken,
         "token_type": "bearer"
       }));