Remove usage of dart:json.

sdk/lib/_internal/pub/test/serve/installs_first_if_source_changed_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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 pub_tests;
 
-import 'dart:json' as json;
+import 'dart:convert';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 main() {
   initConfig();
   integration("installs first if a dependency's source doesn't match the one "
       "in the lock file", () {
     d.dir("foo", [
       d.libPubspec("foo", "0.0.1"),
       d.libDir("foo")
     ]).create();
 
     d.dir(appPath, [
       // A pubspec with a path source.
       d.appPubspec({
         "foo": {"path": "../foo"}
       }),
       // A lock file with the hosted source.
-      d.file("pubspec.lock", json.stringify({
+      d.file("pubspec.lock", JSON.encode({
         'packages': {
           'foo': {
             'version': '0.0.0',
             'source': 'hosted',
             'description': 'foo'
           }
         }
       }))
     ]).create();
 
     startPubServe(shouldInstallFirst: true);
     requestShouldSucceed("packages/foo/foo.dart", 'main() => "foo";');
     endPubServe();
   });
 }