Removed SecurityContext.alpnSupported, as ALPN is now supported on all platforms. Also updated CHAN…

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 8d55c19a2457ed9eeb62f1635a7c42ec2544f149..73f883c76612714454453d9b9a5e6e9074d06677 100644
--- a/sdk/lib/io/security_context.dart
+++ b/sdk/lib/io/security_context.dart
@@ -148,11 +148,6 @@ abstract class SecurityContext {
    */
   void setClientAuthoritiesBytes(List<int> authCertBytes, {String password});
 
-  /**
-   * Whether the platform supports ALPN.
-   */
-  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