Improve the encoding/decoding to/from system encoding on Windows

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;
 
 /// The current system encoding.
+///
+/// This us used for converting from bytes to/from String when
+/// communicating on stdin, stdout and stderr.
+///
+/// On Windows this will use the currently active code page for the
+/// conversion. On all other systems it will always use UTF-8.
 const SystemEncoding SYSTEM_ENCODING = const SystemEncoding();
 
 /**
  * The system encoding is the current code page on Windows and UTF-8 on
  * Linux and Mac.
  */
 class SystemEncoding extends Encoding {
   const SystemEncoding();
 
   String get name => 'system';
@@ skipping 103 matching lines @@
   _WindowsCodePageDecoderSink(this._sink);
 
   void close() {
     _sink.close();
   }
 
   void add(List<int> bytes) {
     _sink.add(_WindowsCodePageDecoder._decodeBytes(bytes));
   }
 }