Fix path tests on windows.

utils/tests/pub/install/path/nonexistent_dir_test.dart

 // Copyright (c) 2013, 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 'dart:io';
 
 import '../../../../../pkg/path/lib/path.dart' as path;
 
 import '../../../../pub/exit_codes.dart' as exit_codes;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
   integration('path dependency to non-existent directory', () {
     var badPath = path.join(sandboxDir, "bad_path");
 
     dir(appPath, [
       pubspec({
         "name": "myapp",
         "dependencies": {
           "foo": {"path": badPath}
         }
       })
     ]).scheduleCreate();
 
+    // 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 '$badPath'."),
+            new RegExp("Could not find package 'foo' at '$escapePath'."),
         exitCode: exit_codes.DATA);
   });
 }