Use helper methods for running pub.

sdk/lib/_internal/pub/test/install/path/relative_symlink_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 'dart:io';
 
 import 'package:pathos/path.dart' as path;
 
 import '../../../lib/src/exit_codes.dart' as exit_codes;
 import '../../descriptor.dart' as d;
@@ skipping 16 matching lines @@
 
     d.dir(appPath, [
       d.pubspec({
         "name": "myapp",
         "dependencies": {
           "foo": {"path": "../foo"}
         }
       })
     ]).create();
 
-    schedulePub(args: ["install"],
-        output: new RegExp(r"Dependencies installed!$"));
+    pubInstall();
 
     d.dir("moved").create();
 
     // Move the app and package. Since they are still next to each other, it
     // should still be found.
     scheduleRename("foo", path.join("moved", "foo"));
     scheduleRename(appPath, path.join("moved", appPath));
 
     d.dir("moved", [
       d.dir(packagesPath, [
         d.dir("foo", [
           d.file("foo.dart", 'main() => "foo";')
         ])
       ])
     ]).validate();
   });
 }