Remove deprecated string features.

tests/standalone/io/file_system_links_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.
 
 import "dart:io";
 import "dart:isolate";
 
 
 createLink(String dst, String link, bool symbolic, void callback()) {
   var args = [ symbolic ? '-s' : '', dst, link ];
@@ skipping 48 matching lines @@
   });
 }
 
 
 testFileWriteRead() {
   var temp = new Directory('').createTempSync();
   var x = '${temp.path}${Platform.pathSeparator}x';
   var y = '${temp.path}${Platform.pathSeparator}y';
   new File(x).createSync();
   createLink(x, y, true, () {
-    var data = "asdf".charCodes;
+    var data = "asdf".codeUnits;
     var output = new File(y).openWrite(FileMode.WRITE);
     output.add(data);
     output.close();
     output.done.then((_) {
       var read = new File(y).readAsBytesSync();
       Expect.listEquals(data, read);
       var read2 = new File(x).readAsBytesSync();
       Expect.listEquals(data, read2);
       temp.deleteSync(recursive: true);
     });
@@ skipping 124 matching lines @@
 
 main() {
   testFileExistsCreate();
   testFileDelete();
   testFileWriteRead();
   testDirectoryExistsCreate();
   testDirectoryDelete();
   testDirectoryListing();
   testDirectoryListingBrokenLink();
 }