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

tests/standalone/io/process_stdin_transform_unsubscribe_script.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.
 //
 // Utility script to echo stdin to stdout or stderr or both.
 
 import "dart:convert";
 import "dart:io";
 
 main() {
   var subscription;
   subscription = stdin
-      .transform(new StringDecoder())
+      .transform(UTF8.decoder)
       .transform(new LineSplitter())
       .listen((String line) {
           // Unsubscribe after the first line.
           subscription.cancel();
         });
 }