Use correct path escape on Windows, for Process.

tests/standalone/io/process_shell_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 "dart:isolate";
 
 void testRunShell() {
   test(args) {
     var options = new Options();
@@ skipping 37 matching lines @@
     var path = new Path(options.script);
     path = path.directoryPath.join(new Path("process_echo_util.dart"));
     Process.run(exe, args, runInShell: true)
         .then((result) {
           port.close();
           if (result.exitCode == 0) {
             throw "error expected";
           }
         });
   }
+  test("'\"'");
   test("'\$HOME'");
 }
 
 void main() {
   testRunShell();
   testBadRunShell();
 }