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

tests/standalone/io/stdin_sync_script.dart

Index: tests/standalone/io/stdin_sync_script.dart
diff --git a/tests/standalone/io/stdin_sync_script.dart b/tests/standalone/io/stdin_sync_script.dart
index d2bc4ad4c2cf615acd73863a6a479d2f32fcc280..ed60a9c4fcda346b8951a822007de2c4fd3dafb9 100644
--- a/tests/standalone/io/stdin_sync_script.dart
+++ b/tests/standalone/io/stdin_sync_script.dart
@@ -2,6 +2,7 @@
 // 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";
 
@@ -9,7 +10,7 @@ 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++;
   }