Use helper methods for running pub.

sdk/lib/_internal/pub/test/install/path/nonexistent_dir_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 'package:scheduled_test/scheduled_test.dart';
 
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('path dependency to non-existent directory', () {
     var badPath = path.join(sandboxDir, "bad_path");
 
     d.dir(appPath, [
       d.pubspec({
         "name": "myapp",
         "dependencies": {
           "foo": {"path": badPath}
         }
       })
     ]).create();
 
-    // TODO(rnystrom): The "\" in a Windows path gets treated like a regex
-    // character, so hack escape. A better fix is to use a literal string
-    // instead of a RegExp to validate, but that requires us to move the
-    // stack traces out of the stderr when we invoke pub. See also: #4706.
-    var escapePath = badPath.replaceAll(r"\", r"\\");
-
-    schedulePub(args: ['install'],
-        error:
-            new RegExp("Could not find package 'foo' at '$escapePath'."),
-        exitCode: 1);
+    pubInstall(error: "Could not find package 'foo' at '$badPath'.");
   });
 }