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

tests/utils/utf_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 utf_test;
 import 'dart:utf';
 
 main() {
-  String str = codepointsToString([0x1d537]);
-  // String.charCodes gives 16-bit code units, but stringToCodepoints gives
-  // back the original code points.
-  Expect.listEquals([0xd835, 0xdd37], str.charCodes);
-  Expect.listEquals([0x1d537], stringToCodepoints(str));
+  String str = new String.fromCharCodes([0x1d537]);
+  // String.charCodes gives the original code points and String.codeUnits
+  // gives back 16-bit code units
+  Expect.listEquals([0xd835, 0xdd37], str.codeUnits);
+  Expect.listEquals([0x1d537], str.charCodes);
 }