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

sdk/lib/_internal/js_runtime/lib/io_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/io_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/io_patch.dart b/sdk/lib/_internal/js_runtime/lib/io_patch.dart
index 2b71c2a1b6829958c6cdfd977f3b363c2007eaad..142495d4cd3831666e90f4351610cb517e4feb44 100644
--- a/sdk/lib/_internal/js_runtime/lib/io_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/io_patch.dart
@@ -459,6 +459,12 @@ class SecurityContext {
   static SecurityContext get defaultContext {
     throw new UnsupportedError("default SecurityContext getter");
   }
+
+  @patch
+  @deprecated
+  static bool get alpnSupported {
+    throw new UnsupportedError("SecurityContext alpnSupported getter");
+  }
 }
 
 @patch