Remove deprecated string features.

pkg/http/lib/src/multipart_request.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.
 
 library multipart_request;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math';
 import 'dart:uri';
@@ skipping 80 matching lines @@
     // TODO(nweiz): freeze fields and files
     var boundary = _boundaryString(_BOUNDARY_LENGTH);
     headers['content-type'] = 'multipart/form-data; boundary="$boundary"';
     headers['content-transfer-encoding'] = 'binary';
     super.finalize();
 
     var controller = new StreamController<List<int>>();
 
     void writeAscii(String string) {
       assert(isPlainAscii(string));
-      controller.add(string.charCodes);
+      controller.add(string.codeUnits);
     }
 
     writeUtf8(String string) => controller.add(encodeUtf8(string));
     writeLine() => controller.add([13, 10]); // \r\n
 
     fields.forEach((name, value) {
       writeAscii('--$boundary\r\n');
       writeAscii(_headerForField(name, value));
       writeUtf8(value);
       writeLine();
@@ skipping 54 matching lines @@
   String _boundaryString(int length) {
     var prefix = "dart-http-boundary-";
     var list = new List<int>.fixedLength(length - prefix.length);
     for (var i = 0; i < list.length; i++) {
       list[i] = _BOUNDARY_CHARACTERS[
           _random.nextInt(_BOUNDARY_CHARACTERS.length)];
     }
     return "$prefix${new String.fromCharCodes(list)}";
   }
 }