Remove deprecated string features.

tests/standalone/io/http_basic_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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 import "dart:isolate";
@@ skipping 210 matching lines @@
     HttpClient httpClient = new HttpClient();
     void sendRequest() {
       HttpClientConnection conn =
           httpClient.post("127.0.0.1", port, "/echo");
       conn.onRequest = (HttpClientRequest request) {
         if (chunkedEncoding) {
           request.outputStream.writeString(data.substring(0, 10));
           request.outputStream.writeString(data.substring(10, data.length));
         } else {
           request.contentLength = data.length;
-          request.outputStream.write(data.charCodes);
+          request.outputStream.write(data.codeUnits);
         }
         request.outputStream.close();
       };
       conn.onResponse = (HttpClientResponse response) {
         Expect.equals(HttpStatus.OK, response.statusCode);
         StringInputStream stream = new StringInputStream(response.inputStream);
         StringBuffer body = new StringBuffer();
         stream.onData = () => body.add(stream.read());
         stream.onClosed = () {
           Expect.equals(data, body.toString());
@@ skipping 61 matching lines @@
 }
 
 void main() {
   testStartStop();
   testGET();
   testPOST(true);
   testPOST(false);
   test404();
   testReasonPhrase();
 }