Remove Encoding-enum from dart:io and add interface in dart:convert.

sdk/lib/io/stdio.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;
 
 const int _STDIO_HANDLE_TYPE_TERMINAL = 0;
 const int _STDIO_HANDLE_TYPE_PIPE = 1;
 const int _STDIO_HANDLE_TYPE_FILE = 2;
 const int _STDIO_HANDLE_TYPE_SOCKET = 3;
@@ skipping 40 matching lines @@
 
   /**
    * Synchronously read a line from stdin. This call will block until a full
    * line is available. The line will contain the newline character(s).
    *
    * If end-of-file is reached, `null` is returned.
    *
    * If end-of-file is reached after some data has already been read, that data
    * is returned.
    */
-  String readLineSync({Encoding encoding: Encoding.SYSTEM,
+  String readLineSync({Encoding encoding: SYSTEM_ENCODING,
                        bool retainNewlines: false}) {
     const CR = 13;
     const LF = 10;
     var line = new StringBuffer();
     bool end = false;
     bool lastCharWasCR = false;
     var error;
 
     StreamController<List<int>> controller =
         new StreamController<List<int>>(sync: true);
-    Stream stream = controller.stream.transform(new StringDecoder(encoding));
+    Stream stream = controller.stream.transform(encoding.decoder);
     stream.listen((String str) {
       line.write(str);
     }, onError: (e) {
       error = e;
     }, onDone: () {
       end = true;
     });
 
     bool empty = true;
     while (!end) {
@@ skipping 164 matching lines @@
   }
   return StdioType.OTHER;
 }
 
 
 class _StdIOUtils {
   external static IOSink _getStdioOutputStream(int fd);
   external static Stdin _getStdioInputStream();
   external static int _socketType(nativeSocket);
 }