Make instances of HeaderValue and ContentType immutable

sdk/lib/io/string_transformer.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.io;
 
 /**
  * String encodings.
  */
 class Encoding {
-  static const Encoding UTF_8 = const Encoding._internal("UTF-8");
-  static const Encoding ISO_8859_1 = const Encoding._internal("ISO-8859-1");
-  static const Encoding ASCII = const Encoding._internal("ASCII");
+  static const Encoding UTF_8 = const Encoding._internal("utf-8");
+  static const Encoding ISO_8859_1 = const Encoding._internal("iso-8859-1");
+  static const Encoding ASCII = const Encoding._internal("us-ascii");
+
+  /**
+   * Name of the encoding. This will be 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)
+   */
+  final String name;
+
   /**
    * SYSTEM encoding is the current code page on Windows and UTF-8 on
    * Linux and Mac.
    */
-  static const Encoding SYSTEM = const Encoding._internal("SYSTEM");
+  static const Encoding SYSTEM = const Encoding._internal("system");
   const Encoding._internal(String this.name);
-  final String name;
 }
 
 
 /**
  * Stream transformer that can decode a stream of bytes into a stream of
  * strings using [encoding].
  *
  * Invalid or forbidden byte-sequences will not produce errors, but will instead
  * insert [replacementChar] in the decoded strings.
  */
@@ skipping 275 matching lines @@
 
 // Utility class for decoding Windows current code page data delivered
 // as a stream of bytes.
 class _WindowsCodePageDecoder extends StreamEventTransformer<List<int>, String> {
   void handleData(List<int> data, StreamSink<String> sink) {
     sink.add(_decodeBytes(data));
   }
 
   external static String _decodeBytes(List<int> bytes);
 }