More String + elimination in tests

tests/standalone/io/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.
 //
 // Directory listing test.
 
 #import("dart:io");
 #import("dart:isolate");
 
 class DirectoryTest {
@@ skipping 274 matching lines @@
     if (new Directory("/tmp").existsSync()) {
       stage0();
     } else {
       emptyTemplateTestRunning = true;
       stage3();
     }
   }
 
   static void testCreateDeleteTemp() {
     new Directory("").createTemp().then((tempDirectory) {
-      String filename = tempDirectory.path +
-          Platform.pathSeparator + "dart_testfile";
+      String filename =
+          "${tempDirectory.path}${Platform.pathSeparator}dart_testfile";
       File file = new File(filename);
       Expect.isFalse(file.existsSync());
       file.create().then((ignore) {
         file.exists().then((exists) {
           Expect.isTrue(exists);
           // Try to delete the directory containing the file - should throw.
           Expect.throws(tempDirectory.deleteSync);
           Expect.isTrue(tempDirectory.existsSync());
 
           // Delete the file, and then delete the directory.
@@ skipping 99 matching lines @@
   future.handleException((e) => port.close());
 }
 
 
 main() {
   DirectoryTest.testMain();
   NestedTempDirectoryTest.testMain();
   testCreateTempErrorSync();
   testCreateTempError();
 }