Remove deprecated string features.

tests/standalone/io/http_read_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
 
 import "dart:isolate";
@@ skipping 156 matching lines @@
     int count = 0;
     HttpClient httpClient = new HttpClient();
     void sendRequest() {
       httpClient.post("127.0.0.1", port, "/echo")
           .then((request) {
             if (chunkedEncoding) {
               request.addString(data.substring(0, 10));
               request.addString(data.substring(10, data.length));
             } else {
               request.contentLength = data.length;
-              request.add(data.charCodes);
+              request.add(data.codeUnits);
             }
             return request.close();
           })
           .then((response) {
             Expect.equals(HttpStatus.OK, response.statusCode);
             List<int> body = new List<int>();
             response.listen(
                 body.addAll,
                 onDone: () {
                   Expect.equals(data, new String.fromCharCodes(body));
@@ skipping 14 matching lines @@
   if (chunkedEncoding) {
     server.chunkedEncoding();
   }
   server.start();
 }
 
 void main() {
   testRead(true);
   testRead(false);
 }