Use "package:" imports for pub.

test/global/activate/missing_package_arg_test.dart

 // Copyright (c) 2014, 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 'package:pub/src/exit_codes.dart' as exit_codes;
 import 'package:scheduled_test/scheduled_test.dart';
 
-import '../../../lib/src/exit_codes.dart' as exit_codes;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('fails if no package was given', () {
     schedulePub(args: ["global", "activate"],
         error: contains("No package to activate given."),
         exitCode: exit_codes.USAGE);
   });
 }