Remove usage of dart:json.

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

Index: sdk/lib/_internal/pub/test/lish/utils.dart
diff --git a/sdk/lib/_internal/pub/test/lish/utils.dart b/sdk/lib/_internal/pub/test/lish/utils.dart
index 70020cfeda2754166f14c3deaa8b487c478b74ee..827928194c8163cc15d0d7b2fe1c2f9c94fef3ce 100644
--- a/sdk/lib/_internal/pub/test/lish/utils.dart
+++ b/sdk/lib/_internal/pub/test/lish/utils.dart
@@ -4,8 +4,8 @@
 
 library lish.utils;
 
+import 'dart:convert';
 import 'dart:io';
-import 'dart:json' as json;
 
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:scheduled_test/scheduled_server.dart';
@@ -30,7 +30,7 @@ void handleUploadForm(ScheduledServer server, [Map body]) {
 
       request.response.headers.contentType =
           new ContentType("application", "json");
-      request.response.write(json.stringify(body));
+      request.response.write(JSON.encode(body));
       request.response.close();
     });
   });