Fix strong mode errors in dart:io.

runtime/bin/socket_patch.dart

Index: runtime/bin/socket_patch.dart
diff --git a/runtime/bin/socket_patch.dart b/runtime/bin/socket_patch.dart
index 8025ab40b47d481f1f40e929e7a572b78e3254b4..fc19bd3cc7fad50b758ccac04ea2f2489b1cd542 100644
--- a/runtime/bin/socket_patch.dart
+++ b/runtime/bin/socket_patch.dart
@@ -46,6 +46,11 @@ patch class InternetAddress {
       String host, {InternetAddressType type: InternetAddressType.ANY}) {
     return _NativeSocket.lookup(host, type: type);
   }
+
+  /* patch */ static InternetAddress _cloneWithNewHost(
+      InternetAddress address, String host) {
+    return (address as _InternetAddress)._cloneWithNewHost(host);
+  }
 }
 
 patch class NetworkInterface {