Don't define parameters when visiting a function-typed formal parameter.

tests/language/issue13179_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:isolate';
 import "package:expect/expect.dart";
 
-main(List<String> args, message) {
-  var sendPort1 = args[0] as SendPort;
-  var sendPort2 = args[1] as SendPort;
-  sendPort2.send(sendPort1);
+int count = 0;
+
+void f([void f([x]) = f]) {
+  count++;
+  if (f != null) {
+    f(null);
+  }
 }
+
+main() {
+  f();
+  Expect.equals(2, count);
+}