Reapply part of r5325.

runtime/bin/directory_impl.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.
 
 
 class _Directory implements Directory {
   static final kCreateRequest = 0;
   static final kDeleteRequest = 1;
   static final kExistsRequest = 2;
   static final kCreateTempRequest = 3;
@@ skipping 56 matching lines @@
   void createTemp(void callback()) {
     _ensureDirectoryService();
     List request = new List(2);
     request[0] = kCreateTempRequest;
     request[1] = _path;
     _directoryService.call(request).then((result) {
       if (result is !List) {
         _path = result;
         callback();
       } else if (_onError != null) {
-        _onError("Could not create temporary directory [$_path]: " +
-                 "${result[1]}");
+        _onError("Could not create temporary directory [$_path]: ${result[1]}");
       }
     });
   }
 
   void createTempSync() {
     if (_path is !String) {
       throw new IllegalArgumentException();
     }
     var result = _createTemp(path);
     if (result is OSError) {
@@ skipping 104 matching lines @@
   }
 
   var _onDir;
   var _onFile;
   var _onDone;
   var _onError;
 
   String _path;
   SendPort _directoryService;
 }