Remove usage of dart:json.

sdk/lib/_internal/pub/test/deploy/copies_browser_js_next_to_entrypoints_test.dart

 // Copyright (c) 2013, 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.
 
-import 'dart:json' as json;
+import 'dart:convert';
 
 import 'package:path/path.dart' as path;
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 main() {
   initConfig();
 
   integration("compiles dart.js and interop.js next to entrypoints", () {
     // Dart2js can take a long time to compile dart code, so we increase the
     // timeout to cope with that.
     currentSchedule.timeout *= 3;
 
     serve([
       d.dir('api', [
         d.dir('packages', [
-          d.file('browser', json.stringify({
+          d.file('browser', JSON.encode({
             'versions': [packageVersionApiMap(packageMap('browser', '1.0.0'))]
           })),
           d.dir('browser', [
             d.dir('versions', [
-              d.file('1.0.0', json.stringify(
+              d.file('1.0.0', JSON.encode(
                   packageVersionApiMap(
                       packageMap('browser', '1.0.0'),
                       full: true)))
             ])
           ])
         ])
       ]),
       d.dir('packages', [
         d.dir('browser', [
           d.dir('versions', [
@@ skipping 49 matching lines @@
             d.file('dart.js', 'contents of dart.js'),
             d.file('interop.js', 'contents of interop.js')
           ])]),
           d.matcherFile('subfile.dart.js', isNot(isEmpty)),
           d.matcherFile('subfile.dart', isNot(isEmpty))
         ])
       ])
     ]).validate();
   });
 }