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

tests/standalone/io/stdin_sync_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.
 
+import "dart:convert";
 import "dart:io";
 import "dart:json";
 
 void main() {
   var arguments = new Options().arguments;
   int i = 0;
   String line;
-  while ((line = stdin.readLineSync(encoding: Encoding.UTF_8)) != null) {
+  while ((line = stdin.readLineSync(encoding: UTF8)) != null) {
     if (parse(arguments[i]) != line) throw "bad line at $i: ${line.codeUnits}";
     i++;
   }
   if (i != arguments.length) throw "expect ${arguments.length} lines";
   print('true');
 }