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

tests/standalone/io/secure_socket_renegotiate_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.
 
 // This test verifies that client certificates work, if the client and server
 // are in separate processes, and that connection renegotiation works, and
 // can request a client certificate to be sent.
 
 import "dart:async";
 import "dart:convert";
@@ skipping 12 matching lines @@
 Future<SecureServerSocket> runServer() {
   SecureSocket.initialize(database: certificateDatabase(),
                           password: 'dartdart');
 
   return SecureServerSocket.bind(HOST_NAME, 0, CERTIFICATE)
     .then((SecureServerSocket server) {
       server.listen((SecureSocket socket) {
         Expect.isNull(socket.peerCertificate);
 
         StreamIterator<String> input =
-            new StreamIterator(socket.transform(new StringDecoder())
+            new StreamIterator(socket.transform(UTF8.decoder)
                                      .transform(new LineSplitter()));
         input.moveNext().then((success) {
           Expect.isTrue(success);
           Expect.equals('first', input.current);
           socket.writeln('first reply');
           return input.moveNext();
         }).then((success) {
           Expect.isTrue(success);
           Expect.equals('renegotiated', input.current);
           Expect.isNull(socket.peerCertificate);
@@ skipping 33 matching lines @@
           if (result.exitCode != 0) {
             print("Client failed, stdout:");
             print(result.stdout);
             print("  stderr:");
             print(result.stderr);
             Expect.fail('Client subprocess exit code: ${result.exitCode}');
           }
         });
     });
 }