Remove usage of dart:json.

sdk/lib/_internal/pub/test/unknown_source_test.dart

 // Copyright (c) 2012, 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 d.file.
 
 library pub_tests;
 
-import 'dart:json' as json;
+import 'dart:convert';
 
 import 'descriptor.dart' as d;
 import 'test_pub.dart';
 
 main() {
   initConfig();
 
   forBothPubInstallAndUpdate((command) {
     integration('fails gracefully on a dependency from an unknown source', () {
       d.appDir({"foo": {"bad": "foo"}}).create();
@@ skipping 23 matching lines @@
 
       // Depend on "foo" from a valid source.
       d.dir(appPath, [
         d.appPubspec({
           "foo": {"path": "../foo"}
         })
       ]).create();
 
       // But lock it to a bad one.
       d.dir(appPath, [
-        d.file("pubspec.lock", json.stringify({
+        d.file("pubspec.lock", JSON.encode({
           'packages': {
             'foo': {
               'version': '0.0.0',
               'source': 'bad',
               'description': {
                 'name': 'foo'
               }
             }
           }
         }))
       ]).create();
 
       pubCommand(command);
 
       // Should update to the new one.
       d.dir(packagesPath, [
         d.dir("foo", [
           d.file("foo.dart", 'main() => "foo";')
         ])
       ]).validate();
     });
   });
 }