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

tests/standalone/io/regress_10026_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 "package:expect/expect.dart";
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 import 'dart:isolate';
 
 void testZLibInflate_regress10026() {
   test(data, expect) {
     var port = new ReceivePort();
     var controller = new StreamController(sync: true);
     controller.stream
         .transform(ZLIB.decoder)
-        .transform(new StringDecoder())
+        .transform(UTF8.decoder)
         .fold(new StringBuffer(), (buffer, s) {
           buffer.write(s);
           return buffer;
         })
         .then((out) {
           Expect.equals(out.toString(), expect);
           port.close();
         });
     controller.add(data);
     controller.close();
@@ skipping 103 matching lines @@
 </div>
 </body>
 </html>
 ''');
 }
 
 void main() {
   testZLibInflate_regress10026();
 }