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

tests/lib/codec/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 '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));
-
-  Expect.listEquals([], UTF8.encode(""));
-  Expect.equals("", UTF8.decode([]));
-
-  final JSON_ENCODED = RAW;
-  Expect.equals(JSON_ENCODED, JSON.encode(["122ç", 50, 50, 231]));
-  Expect.listEquals(["122ç", 50, 50, 231], JSON.decode(JSON_ENCODED));
-
-  // Test that the reviver is passed to the decoder.
-  var decoded = JSON.decode('{"p": 5}', reviver: (k, v) {
-    if (k == "") return v;
-    return v * 2;
-  });
-  Expect.equals(10, decoded["p"]);
-  var jsonWithReviver = new JsonCodec.withReviver((k, v) {
-    if (k == "") return v;
-    return v * 2;
-  });
-  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]);
-}