Use "package:" imports for pub.

test/list_package_dirs/missing_pubspec_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 d.file.
 
 import 'package:path/path.dart' as path;
+import 'package:pub/src/io.dart';
 
-import '../../lib/src/io.dart';
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 main() {
   initConfig();
   // This is a regression test for #20065.
   integration("reports a missing pubspec error using JSON", () {
     d.dir(appPath).create();
 
     schedulePub(args: ["list-package-dirs", "--format=json"],
         outputJson: {
           "error":
             'Could not find a file named "pubspec.yaml" in "'
                 '${canonicalize(path.join(sandboxDir, appPath))}".',
           "path": canonicalize(path.join(sandboxDir, appPath, "pubspec.yaml"))
         },
         exitCode: 1);
   });
 }