More String + elimination in tests

tests/standalone/io/process_working_directory_test.dart

 // Copyright (c) 2012, 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.
 //
 // Process working directory test.
 
 #library("ProcessWorkingDirectoryTest");
 #import("dart:io");
 #source("process_test_util.dart");
 
@@ skipping 25 matching lines @@
       Expect.fail("error running process $error");
       directory.deleteSync();
     };
   }
 
   static void testInvalidDirectory() {
     Directory directory = new Directory("").createTempSync();
     Expect.isTrue(directory.existsSync());
 
     var options = new ProcessOptions();
-    options.workingDirectory = directory.path + "/subPath";
+    options.workingDirectory = directory.path.concat("/subPath");
     Process process = Process.start(fullTestFilePath,
                                     const ["0", "0", "99", "0"],
                                     options);
 
     process.onExit = (int exitCode) {
       Expect.fail("bad process completed");
       process.close();
       directory.deleteSync();
     };
 
     process.onError = (error) {
       Expect.isNotNull(error);
       directory.deleteSync();
     };
   }
 }
 
 
 
 main() {
   ProcessWorkingDirectoryTest.testValidDirectory();
   ProcessWorkingDirectoryTest.testInvalidDirectory();
 }