Implements secure sockets on Mac OS with SecureTransport API

tests/standalone/io/secure_socket_alpn_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 'dart:io';
 import 'dart:convert';
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 
@@ skipping 95 matching lines @@
       Expect.isTrue(e is ArgumentError);
       Expect.isTrue(e.toString().contains(errorIncludes));
       server.close();
       asyncEnd();
     });
     asyncEnd();
   });
 }
 
 main() {
+  if (!SecurityContext.alpnSupported) {
+    return 0;
+  }
   final longname256 = 'p' * 256;
   final String longname255 = 'p' * 255;
   final String strangelongname255 = 'ø' + 'p' * 253;
   final String strangelongname256 = 'ø' + 'p' * 254;
 
   // This produces a message of (1 << 13) - 2 bytes. 2^12 -1 strings are each
   // encoded by 1 length byte and 1 ascii byte.
   final List<String> manyProtocols = new Iterable.generate(
       (1 << 12) - 1, (i) => '0').toList();
 
@@ skipping 69 matching lines @@
   // Issue  https://github.com/dart-lang/sdk/issues/23580
   // Chromium issue https://code.google.com/p/chromium/issues/detail?id=497770
   testSuccessfulAlpnNegotiationConnection(['a'], ['b'], null);
 
   // Test too short / too long protocol names.
   testInvalidArgument([longname256], NAME_LENGTH_ERROR);
   testInvalidArgument([strangelongname256], NAME_LENGTH_ERROR);
   testInvalidArgument([''], NAME_LENGTH_ERROR);
   testInvalidArgument(tooManyProtocols, MESSAGE_LENGTH_ERROR);
 }