Rename String.concat to operator+.

tests/standalone/io/pipe_server_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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 library ServerTest;
@@ skipping 35 matching lines @@
 
   void runTest() {
 
     void connectHandler() {
       String srcFileName =
           getDataFilename("tests/standalone/io/readline_test1.dat");
       Stream fileInput = new File(srcFileName).openRead();
 
       fileInput.pipe(_socket).then((_) {
         var tempDir = new Directory('').createTempSync();
-        var dstFileName = tempDir.path.concat("/readline_test1.dat");
+        var dstFileName = tempDir.path + "/readline_test1.dat";
         var dstFile = new File(dstFileName);
         dstFile.createSync();
         var fileOutput = dstFile.openWrite();
         _socket.pipe(fileOutput).then((_) {
           // Check that the resulting file is equal to the initial
           // file.
           bool result = compareFileContent(srcFileName, dstFileName);
           new File(dstFileName).deleteSync();
           tempDir.deleteSync();
           Expect.isTrue(result);
@@ skipping 47 matching lines @@
 class PipeServer extends TestingServer {
   void onConnection(Socket connection) {
     connection.pipe(connection);
   }
 }
 
 
 main() {
   PipeServerGame echoServerGame = new PipeServerGame.start();
 }