Remove dart:codec and move classes into dart:convert.

tests/lib/convert/codec2_test.dart

 // Copyright (c) 2013, 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:codec';
+import 'dart:convert';
 
 import 'package:expect/expect.dart';
 
 main() {
   final RAW = '["122ç",50,50,231]';
   final ENCODED = const [91, 34, 49, 50, 50, 195, 167, 34, 44,
                          53, 48, 44, 53, 48, 44, 50, 51, 49, 93];
   Expect.listEquals(ENCODED, UTF8.encode(RAW));
   Expect.equals(RAW, UTF8.decode(ENCODED));
 
@@ skipping 17 matching lines @@
   decoded = jsonWithReviver.decode('{"p": 5}');
   Expect.equals(10, decoded["p"]);
 
   // Test example from comments.
   final JSON_TO_BYTES = JSON.fuse(UTF8);
   List<int> bytes = JSON_TO_BYTES.encode(["json-object"]);
   decoded = JSON_TO_BYTES.decode(bytes);
   Expect.isTrue(decoded is List);
   Expect.equals("json-object", decoded[0]);
 }