Replace old utf8 decoder with new one.

tests/lib/convert/utf8_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 utf8_test;
 import "package:expect/expect.dart";
-import 'dart:utf';
+import 'dart:convert';
 
-String decode(List<int> bytes) => decodeUtf8(bytes);
+String decode(List<int> bytes) => new Utf8Decoder().convert(bytes);
 
 main() {
   // Google favorite: "Îñţérñåţîöñåļîžåţîờñ".
   String string = decode([0xc3, 0x8e, 0xc3, 0xb1, 0xc5, 0xa3, 0xc3, 0xa9, 0x72,
                           0xc3, 0xb1, 0xc3, 0xa5, 0xc5, 0xa3, 0xc3, 0xae, 0xc3,
                           0xb6, 0xc3, 0xb1, 0xc3, 0xa5, 0xc4, 0xbc, 0xc3, 0xae,
                           0xc5, 0xbe, 0xc3, 0xa5, 0xc5, 0xa3, 0xc3, 0xae, 0xe1,
                           0xbb, 0x9d, 0xc3, 0xb1]);
   Expect.stringEquals("Îñţérñåţîöñåļîžåţîờñ", string);
 
@@ skipping 18 matching lines @@
 
   // DESERET CAPITAL LETTER BEE, unicode 0x10412(0xD801+0xDC12)
   // UTF-8: F0 90 90 92
   string = decode([0xf0, 0x90, 0x90, 0x92]);
   Expect.equals(string.length, 2);
   Expect.equals("𐐒".length, 2);
   Expect.stringEquals("𐐒", string);
 
   // TODO(ahe): Add tests of bad input.
 }