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

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 73f883c76612714454453d9b9a5e6e9074d06677..24e2719f4194b166fcb01602330f369809936679 100644
--- a/sdk/lib/io/security_context.dart
+++ b/sdk/lib/io/security_context.dart
@@ -148,6 +148,12 @@ abstract class SecurityContext {
    */
   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