Implements secure sockets on Mac OS with SecureTransport API

runtime/bin/secure_socket_boringssl.h

Index: runtime/bin/secure_socket_boringssl.h
diff --git a/runtime/bin/secure_socket.h b/runtime/bin/secure_socket_boringssl.h
similarity index 94%
copy from runtime/bin/secure_socket.h
copy to runtime/bin/secure_socket_boringssl.h
index 70a24378395171a258510dfc109bd78b8fe26d5a..9dbe25083206f9a221d2992e143d1242ffd4c894 100644
--- a/runtime/bin/secure_socket.h
+++ b/runtime/bin/secure_socket_boringssl.h
@@ -2,11 +2,11 @@
 // 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.
 
-#ifndef BIN_SECURE_SOCKET_H_
-#define BIN_SECURE_SOCKET_H_
+#ifndef BIN_SECURE_SOCKET_BORINGSSL_H_
+#define BIN_SECURE_SOCKET_BORINGSSL_H_
 
-#ifdef DART_IO_SECURE_SOCKET_DISABLED
-#error "secure_socket.h can only be included on builds with SSL enabled"
+#if !defined(BIN_SECURE_SOCKET_H_)
+#error Do not include secure_socket_boringssl.h directly. Use secure_socket.h.
 #endif
 
 #include <stdio.h>
@@ -130,4 +130,4 @@ class SSLFilter {
 }  // namespace bin
 }  // namespace dart
 
-#endif  // BIN_SECURE_SOCKET_H_
+#endif  // BIN_SECURE_SOCKET_BORINGSSL_H_