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

tests/lib/convert/codec1_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';
 
 class MyCodec extends Codec<int, String> {
   const MyCodec();
 
   Converter<int, String> get encoder => new IntStringConverter();
   Converter<String, int> get decoder => new StringIntConverter();
 }
@@ skipping 45 matching lines @@
 
   inverted = TEST_CODEC2.inverted;
   fused = TEST_CODEC2.fuse(inverted);
   Expect.equals(499, fused.encode(0));
   Expect.equals(499, fused.decode(0));
 
   fused = TEST_CODEC.fuse(inverted);
   Expect.equals(405, fused.encode(5));
   Expect.equals(101, fused.decode(2));
 }