Use "package:" imports for pub.

test/barback/fails_on_disallowed_directories_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:path/path.dart' as path;
 import 'package:scheduled_test/scheduled_test.dart';
 
-import '../../lib/src/exit_codes.dart' as exit_codes;
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 main() {
   initConfig();
 
   setUp(() {
     d.appDir().create();
   });
 
   var libSub = path.join("lib", "sub");
   pubBuildAndServeShouldFail("if given directories are not allowed",
       args: [libSub, "lib"],
       error: 'Directories "$libSub" and "lib" are not allowed.',
       exitCode: exit_codes.USAGE);
 }