Rename String.concat to operator+.

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";
 import "process_test_util.dart";
 
@@ skipping 24 matching lines @@
       directory.deleteSync();
       Expect.fail("Couldn't start process");
     });
   }
 
   static void testInvalidDirectory() {
     Directory directory = new Directory("").createTempSync();
     Expect.isTrue(directory.existsSync());
 
     var options = new ProcessOptions();
-    options.workingDirectory = directory.path.concat("/subPath");
+    options.workingDirectory = directory.path + "/subPath";
     var future = Process.start(fullTestFilePath,
                                const ["0", "0", "99", "0"],
                                options);
     future.then((process) {
       Expect.fail("bad process completed");
       directory.deleteSync();
     }).catchError((e) {
       Expect.isNotNull(e);
       directory.deleteSync();
     });
   }
 }
 
 
 
 main() {
   ProcessWorkingDirectoryTest.testValidDirectory();
   ProcessWorkingDirectoryTest.testInvalidDirectory();
 }