Implements secure sockets on Mac OS with SecureTransport API

tests/standalone/io/secure_builtin_roots_test.dart

 // Copyright (c) 2012, 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:io";
 import "dart:async";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
 Future testGoogleUrl(SecurityContext context, String outcome) async {
   var client = new HttpClient(context: context);
   // We need to use an external server that is backed by a
   // built-in root certificate authority.
   try {
-  // First, check if the lookup works.
+    // First, check if the lookup works.
     await InternetAddress.lookup('www.google.com');
     var request = await client.getUrl(Uri.parse('https://www.google.com'));
     request.followRedirects = false;
     var response = await request.close();
     Expect.equals('pass', outcome, 'Unexpected successful connection');
     try { await response.drain(); } catch (e) { }
   } on HandshakeException {
     Expect.equals('fail', outcome, 'Unexpected failed connection');
   } on SocketException {
     // Lookup failed or connection failed.  Don't report a failure.
   } finally {
     client.close();
   }
 }
 
 
 main() async {
   asyncStart();
   await testGoogleUrl(null, "pass");
   await testGoogleUrl(SecurityContext.defaultContext, "pass");
   await testGoogleUrl(new SecurityContext(), "fail");
   asyncEnd();
 }