Revert "Removed SecurityContext.alpnSupported, as ALPN is now supported on all platforms. Also upda…

sdk/lib/io/security_context.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.
 
 part of dart.io;
 
 /**
  * The object containing the certificates to trust when making
  * a secure client connection, and the certificate chain and
  * private key to serve from a secure server.
@@ skipping 131 matching lines @@
   /**
    * Sets the list of authority names that a [SecureServerSocket] will advertise
    * as accepted, when requesting a client certificate from a connecting
    * client.
    *
    * Like [setClientAuthorities] but takes the contents of the file.
    */
   void setClientAuthoritiesBytes(List<int> authCertBytes, {String password});
 
   /**
+   * Whether the platform supports ALPN.

[kevmoo, 2017/06/08 20:47:54]

It's good to put a note in the doccomment, too – explain that this always
returns `true` and that it will be removed in a future release.

+   */
+  @deprecated
+  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
    * protocols in the TLS client hello message, and the server to pick
    * one and send the selected one back in its server hello message.
    *
    * Separate lists of protocols can be sent for client connections and
    * for server connections, using the same SecurityContext.  The [isServer]
    * boolean argument specifies whether to set the list for server connections
    * or client connections.
@@ skipping 91 matching lines @@
     }
 
     if (bytes.length >= (1 << 13)) {
       throw new ArgumentError(
           'The maximum message length supported is 2^13-1.');
     }
 
     return new Uint8List.fromList(bytes);
   }
 }