Remove deprecated string features.

tests/lib/crypto/base64_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.
 
 // Library tag to allow the test to run on Dartium.
 library base64_test;
 
 import 'dart:crypto';
 
 // Data from http://tools.ietf.org/html/rfc4648.
@@ skipping 27 matching lines @@
     "BhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2Yg"
     "dGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcm"
     "FuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY29udGlu"
     "dWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYX"
     "Rpb24gb2Yga25vd2xlZGdlLCBleGNlZWRzIHRo"
     "ZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm"
     "5hbCBwbGVhc3VyZS4=";
 
 void main() {
   for (var i = 0; i < inputs.length; i++) {
-    var enc = CryptoUtils.bytesToBase64(inputs[i].charCodes);
+    var enc = CryptoUtils.bytesToBase64(inputs[i].codeUnits);
     Expect.equals(results[i], enc);
   }
-  Expect.equals(CryptoUtils.bytesToBase64(longLine.charCodes, 76),
+  Expect.equals(CryptoUtils.bytesToBase64(longLine.codeUnits, 76),
                 longLineResult);
-  Expect.equals(CryptoUtils.bytesToBase64(longLine.charCodes),
+  Expect.equals(CryptoUtils.bytesToBase64(longLine.codeUnits),
                 longLineResultNoBreak);
 }