Add support for client certificates to Secure[Server]Socket

tests/standalone/io/secure_server_client_no_certificate_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 "dart:async";
-import "dart:io";
-
-import "package:async_helper/async_helper.dart";
-import "package:expect/expect.dart";
-
-InternetAddress HOST;
-String localFile(path) => Platform.script.resolve(path).toFilePath();
-
-SecurityContext serverContext = new SecurityContext()
-  ..useCertificateChain(localFile('certificates/server_chain.pem'))
-  ..usePrivateKey(localFile('certificates/server_key.pem'),
-                  password: 'dartdart');
-
-SecurityContext clientContext = new SecurityContext()
-  ..setTrustedCertificates(file: localFile('certificates/trusted_certs.pem'));
-
-Future testNoClientCertificate() {
-  var completer = new Completer();
-  SecureServerSocket.bind(HOST,
-                          0,
-                          serverContext,
-                          requestClientCertificate: true).then((server) {
-    var clientEndFuture = SecureSocket.connect(HOST,
-                                               server.port,
-                                               context: clientContext);
-    server.listen((serverEnd) {
-      X509Certificate certificate = serverEnd.peerCertificate;
-      Expect.isNull(certificate);
-      clientEndFuture.then((clientEnd) {
-        clientEnd.close();
-        serverEnd.close();
-        server.close();
-        completer.complete();
-      });
-    });
-  });
-  return completer.future;
-}
-
-Future testNoRequiredClientCertificate() {
-  var completer = new Completer();
-  bool clientError = false;
-  SecureServerSocket.bind(HOST,
-                          0,
-                          serverContext,
-                          requireClientCertificate: true).then((server) {
-    Future clientDone =
-        SecureSocket.connect(HOST, server.port, context: clientContext)
-        .catchError((e) { clientError = true; });
-    server.listen((serverEnd) {
-      Expect.fail("Got a unverifiable connection");
-    },
-    onError: (e) {
-      clientDone.then((_) {
-        Expect.isTrue(clientError);
-        server.close();
-        completer.complete();
-      });
-    });
-  });
-  return completer.future;
-}
-
-void main() {
-  asyncStart();
-  InternetAddress.lookup("localhost").then((hosts) => HOST = hosts.first)
-    .then((_) => testNoRequiredClientCertificate())
-    .then((_) => testNoClientCertificate())
-    .then((_) => asyncEnd());
-}