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

sdk/lib/convert/encoding.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.
+
+part of dart.convert;
+
+/**
+ * Open-ended Encoding enum.
+ */
+// TODO(floitsch): dart:io already has an Encoding class. If we can't
+// consolitate them, we need to remove `Encoding` here.
+abstract class Encoding extends Codec<String, List<int>> {
+  const Encoding();
+}
+
+// TODO(floitsch): add other encodings, like ASCII and ISO_8859_1.