Implements secure sockets on Mac OS with SecureTransport API

tests/standalone/io/security_context_argument_test.dart

 // Copyright (c) 2015, 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:io";
 
 String localFile(path) => Platform.script.resolve(path).toFilePath();
 
 bool printException(e) { print(e); return true; }
@@ skipping 98 matching lines @@
         tlsException);
     Expect.throws(() => c.useCertificateChain(
         localFile('certificates/server_chain_malformed1.pem')),
         tlsException);
     Expect.throws(() => c.useCertificateChain(
         localFile('certificates/server_chain_malformed2.pem')),
         tlsException);
     Expect.throws(() => c.setClientAuthorities(
         localFile('certificates/client_authority_malformed.pem')),
         tlsException);
-
-    c.usePrivateKey(
-        localFile('certificates/server_key.pem'), password: "dartdart");
 }
 
 void main() {
   testUsePrivateKeyArguments();
 }