Begin work on ios secure sockets

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.
  *
  * The [SecureSocket]  and [SecureServer] classes take a SecurityContext
  * as an argument to their connect and bind methods.
  *
  * Certificates and keys can be added to a SecurityContext from either PEM
  * or PKCS12 containers.
+ *
+ * iOS note: methods to add, remove, and inspect certificates are not yet
+ * implemented. That is, only the platform's built-in trusted certificates can
+ * be used, by way of [SecurityContext.defaultContext].
  */
 abstract class SecurityContext {
   external factory SecurityContext();
 
   /**
    * Secure networking classes with an optional `context` parameter
    * use the [defaultContext] object if the parameter is omitted.
    * This object can also be accessed, and modified, directly.
    * Each isolate has a different [defaultContext] object.
    * The [defaultContext] object uses a list of well-known trusted
-   * certificate authorities as its trusted roots.  This list is
-   * taken from Mozilla, who maintains it as part of Firefox.
+   * certificate authorities as its trusted roots. On Linux and Windows, this
+   * list is taken from Mozilla, who maintains it as part of Firefox. On,
+   * MacOS, iOS, and Android, this list comes from the trusted certificates
+   * stores built in to the platforms.
    */
   external static SecurityContext get defaultContext;
 
   /**
    * Sets the private key for a server certificate or client certificate.
    *
    * A secure connection using this SecurityContext will use this key with
    * the server or client certificate to sign and decrypt messages.
    * [file] is the path to a PEM or PKCS12 file containing an encrypted
    * private key, encrypted with [password]. Assuming it is well-formatted, all
@@ skipping 196 matching lines @@
     }
 
     if (bytes.length >= (1 << 13)) {
       throw new ArgumentError(
           'The maximum message length supported is 2^13-1.');
     }
 
     return new Uint8List.fromList(bytes);
   }
 }