Implements secure sockets on Mac OS with SecureTransport API

sdk/lib/io/security_context.dart

Index: sdk/lib/io/security_context.dart
diff --git a/sdk/lib/io/security_context.dart b/sdk/lib/io/security_context.dart
index 86dfc965e69ad0da25cf42c07e9e05c6c4aa4f2c..1e169af9d6e7ce476abfb9052e2124337f19272e 100644
--- a/sdk/lib/io/security_context.dart
+++ b/sdk/lib/io/security_context.dart
@@ -126,6 +126,11 @@ abstract class SecurityContext {
   void setClientAuthoritiesBytes(List<int> authCertBytes, {String password});
 
   /**
+   * Whether the platform supports ALPN.
+   */
+  external static bool get alpnSupported;
+
+  /**
    * Sets the list of application-level protocols supported by a client
    * connection or server connection. The ALPN (application level protocol
    * negotiation) extension to TLS allows a client to send a list of