Use "package:" imports for pub.

test/upgrade/dry_run_does_not_apply_changes_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:path/path.dart' as path;
+import 'package:pub/src/io.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
-import '../../lib/src/io.dart';
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 main() {
   initConfig();
   integration("--dry-run shows report but does not apply changes", () {
     servePackages((builder) {
       builder.serve("foo", "1.0.0");
       builder.serve("foo", "2.0.0");
     });
@@ skipping 22 matching lines @@
     ]));
 
     d.dir(appPath, [
       // The lockfile should be unmodified.
       d.matcherFile("pubspec.lock", contains("1.0.0")),
       // The "packages" directory should not have been regenerated.
       d.nothing("packages")
     ]).validate();
   });
 }