Remove deprecated string features.

tests/standalone/io/secure_session_resume_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.
 
 // This test tests TLS session resume, by making multiple client connections
 // on the same port to the same server, with a delay of 200 ms between them.
 // The unmodified secure_server_test creates all sessions simultaneously,
 // which means that no handshake completes and caches its keys in the session
 // cache in time for other connections to use it.
 //
 // Session resume is currently disabled - see issue
 // https://code.google.com/p/dart/issues/detail?id=7230
 
 import "dart:async";
 import "dart:io";
 import "dart:isolate";
 
 const SERVER_ADDRESS = "127.0.0.1";
 const HOST_NAME = "localhost";
 
 void WriteAndClose(Socket socket, String message) {
-  var data = message.charCodes;
+  var data = message.codeUnits;
   int written = 0;
   void write() {
     written += socket.writeList(data, written, data.length - written);
     if (written < data.length) {
       socket.onWrite = write;
     } else {
       socket.close(true);
     }
   }
   write();
@@ skipping 93 matching lines @@
   Duration delay = const Duration(milliseconds: 0);
   Duration delay_between_connections = const Duration(milliseconds: 300);
 
   for (var x in CLIENT_NAMES) {
     new Timer(delay, () {
       new SecureTestClient(port, x);
     });
     delay += delay_between_connections;
   }
 }