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

tests/standalone/io/secure_socket_renegotiate_client.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.
 
 // Client for secure_socket_renegotiate_test, that runs in a subprocess.
 // The test verifies that client certificates work, if the client and server
 // are in separate processes, and that connection renegotiation can request
 // a client certificate to be sent.
 
 import "dart:async";
@@ skipping 26 matching lines @@
 
 
 void runClient(int port) {
   SecureSocket.connect(HOST_NAME, port, sendClientCertificate: true)
     .then((SecureSocket socket) {
       X509Certificate certificate = socket.peerCertificate;
       expect(certificate != null);
       expectEquals('CN=localhost', certificate.subject);
       expectEquals('CN=myauthority', certificate.issuer);
       StreamIterator<String> input = new StreamIterator(socket
-          .transform(new StringDecoder())
+          .transform(UTF8.decoder)
           .transform(new LineSplitter()));
       socket.writeln('first');
       input.moveNext()
         .then((success) {
           expect(success);
           expectEquals('first reply', input.current);
           socket.renegotiate();
           socket.writeln('renegotiated');
           return input.moveNext();
         })
@@ skipping 13 matching lines @@
         });
     });
 }
 
 
 void main() {
   final args = new Options().arguments;
   SecureSocket.initialize(database: args[1], password: 'dartdart');
   runClient(int.parse(args[0]));
 }