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

tests/standalone/io/stdin_sync_test.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.
 
 import "dart:convert";
 import "dart:io";
 import "dart:json";
 
 import "package:path/path.dart";
 
 void testReadByte() {
   void test(String line, List<String> expected) {
     var script = join(dirname(Platform.script), "stdin_sync_script.dart");
     Process.start(Platform.executable,
                   ["--checked", script]..addAll(
                       expected.map(stringify))).then((process) {
       process.stdin.write(line);
       process.stdin.close();
       process.stderr
-          .transform(new StringDecoder())
+          .transform(UTF8.decoder)
           .transform(new LineSplitter())
           .fold(new StringBuffer(), (b, d) => b..write(d))
           .then((data) {
             if (data.toString() != '') throw "Bad output: '$data'";
           });
       process.stdout
-          .transform(new StringDecoder())
+          .transform(UTF8.decoder)
           .transform(new LineSplitter())
           .fold(new StringBuffer(), (b, d) => b..write(d))
           .then((data) {
             if (data.toString() != 'true') throw "Bad output: '$data'";
           });
     });
   }
 
   test("hej\x01\x00\x0d\x0a\x0a4\x0a", ['hej\x01\x00', '', '4']);
 
@@ skipping 24 matching lines @@
 
 
 void main() {
   testReadByte();
 
   // testEchoMode and testLineMode is an developer-interactive test, thus not
   // enabled.
   //testEchoMode();
   //testLineMode();
 }