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..40a2f160f50977b66e38b2aed0ecfef73656d269 100644
--- a/sdk/lib/io/security_context.dart
+++ b/sdk/lib/io/security_context.dart
@@ -148,6 +148,13 @@ abstract class SecurityContext {
    */
   void setClientAuthoritiesBytes(List<int> authCertBytes, {String password});
 
+  /**
+   * Whether the platform supports ALPN. This always returns true and 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