Remove usage of dart:json.

sdk/lib/_internal/pub/lib/src/command/list_package_dirs.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.
 
 library pub.command.list_package_dirs;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
-import 'dart:json' as json;
 
 import 'package:path/path.dart' as path;
 
 import '../command.dart';
 import '../exit_codes.dart' as exit_codes;
 import '../log.dart' as log;
 
 /// Handles the `list-package-dirs` pub command.
 class ListPackageDirsCommand extends PubCommand {
   String get description => "Print local paths to dependencies.";
   String get usage => "pub list-package-dirs";
   bool get hidden => true;
 
   ListPackageDirsCommand() {
     commandParser.addOption("format",
         help: "How output should be displayed.",
         allowed: ["json"]);
   }
 
   Future onRun() {
     if (!entrypoint.lockFileExists) {
-      log.error(json.stringify(
+      log.error(JSON.encode(
           'Package "myapp" has no lockfile. Please run "pub install" first.'));
       exit(exit_codes.NO_INPUT);
     }
 
     var output = {};
 
     // Include the local paths to all locked packages.
     var packages = {};
     var futures = [];
     entrypoint.loadLockFile().packages.forEach((name, package) {
       var source = entrypoint.cache.sources[package.source];
       futures.add(source.getDirectory(package).then((packageDir) {
         packages[name] = path.join(packageDir, "lib");
       }));
     });
 
     output["packages"] = packages;
 
     // Include the self link.
     packages[entrypoint.root.name] = path.join(entrypoint.root.dir, "lib");
 
     // Include the file(s) which when modified will affect the results. For pub,
     // that's just the lockfile.
     output["input_files"] = [entrypoint.lockFilePath];
 
     return Future.wait(futures).then((_) {
-      log.message(json.stringify(output));
+      log.message(JSON.encode(output));
     });
   }
 }