Remove Encoding-enum from dart:io and add interface in 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();
+abstract class Encoding extends Codec<String, List<int>> {
+  const Encoding();
 
-  // TODO(floitsch): should we introduce a StringToByteEncoder and
-  // a ByteToStringDecoder so that we have better typing?
+  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.
+   *
+   * If the encoding is standardized, this is the lower-case version of one of
+   * the IANA official names for the character set (see
+   * http://www.iana.org/assignments/character-sets/character-sets.xml)
+   */
+  String get name;
 }
-
-// TODO(floitsch): add other encodings, like ASCII and ISO_8859_1.