Make convert library strong-mode compliant.

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.
  */
 abstract class Encoding extends Codec<String, List<int>> {
   const Encoding();
 
+  ChunkedConverter<String, List<int>, String, List<int>> get encoder;
+  ChunkedConverter<List<int>, String, List<int>, String> get decoder;
+
   Future<String> decodeStream(Stream<List<int>> byteStream) {
     return byteStream
       .transform(decoder)
       .fold(new StringBuffer(), (buffer, string) => buffer..write(string))
       .then((buffer) => buffer.toString());
   }
 
   /**
    * Name of the encoding.
    *
@@ skipping 44 matching lines @@
   * The [name] passed is case insensitive.
   *
   * If character set is not supported [:null:] is returned.
   */
   static Encoding getByName(String name) {
     if (name == null) return null;
     name = name.toLowerCase();
     return _nameToEncoding[name];
   }
 }