Add some support for the code-point code-unit distinction.

tests/standalone/io/string_stream_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");
 
 void testUtf8() {
   List<int> data = [0x01,
                     0x7f,
@@ skipping 11 matching lines @@
     Expect.equals(8, s.length);
     Expect.equals(new String.fromCharCodes([0x01]), s[0]);
     Expect.equals(new String.fromCharCodes([0x7f]), s[1]);
     Expect.equals(new String.fromCharCodes([0x80]), s[2]);
     Expect.equals(new String.fromCharCodes([0x7ff]), s[3]);
     Expect.equals(new String.fromCharCodes([0x800]), s[4]);
     Expect.equals(new String.fromCharCodes([0xffff]), s[5]);
     Expect.equals(new String.fromCharCodes([0xffff]), s[5]);
 
     // Surrogate pair for U+1D11E.
-    Expect.equals(new String.fromCharCodes([0xd834]), s[6]);
-    Expect.equals(new String.fromCharCodes([0xdd1e]), s[7]);
+    Expect.equals(new String.fromCodeUnits([0xd834, 0xdd1e]),
+                  s.substring(6, 8));
   }
   stream.onData = stringData;
 }
 
 void testLatin1() {
   List<int> data = [0x01,
                     0x7f,
                     0x44, 0x61, 0x72, 0x74,
                     0x80,
                     0xff];
@@ skipping 282 matching lines @@
   testUtf8();
   testLatin1();
   testAscii();
   testReadLine1();
   testReadLine2();
   testReadChunks();
   testReadMixed();
   testErrorHandler();
   testEncodingErrorWithHandler();
 }