Fix strong mode errors in dart:io.

sdk/lib/io/socket.dart

Index: sdk/lib/io/socket.dart
diff --git a/sdk/lib/io/socket.dart b/sdk/lib/io/socket.dart
index 30e4eb2c2dda9d357bf466b1a8c71499aecb7ab4..f85a5bd57825b8a87548f58df9c5a6bf8105fd0c 100644
--- a/sdk/lib/io/socket.dart
+++ b/sdk/lib/io/socket.dart
@@ -141,6 +141,15 @@ abstract class InternetAddress {
    */
   external static Future<List<InternetAddress>> lookup(
       String host, {InternetAddressType type: InternetAddressType.ANY});
+
+  /**
+   * Clones the given [address] with the new [host].
+   *
+   * The [address] must be an [InternetAddress] that was created with one
+   * of the static methods of this class.
+   */
+  external static InternetAddress _cloneWithNewHost(
+      InternetAddress address, String host);
 }