Fix issue 18435 (2nd attempt).

tests/language/regress_18435_test.dart

 // Copyright (c) 2014, 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.
 
 // Regression test for issue 18435.
 
 import "package:expect/expect.dart";
 
 main() {
   const MISSING_VALUE = "MISSING_VALUE";
 
   void foo([var p1 = MISSING_VALUE, var p2 = MISSING_VALUE]) {
     Expect.equals("P1", p1);
     Expect.equals("P2", p2);
   }
 
   void bar([var p1 = "MISSING_VALUE", var p2 = "MISSING_VALUE"]) {
     Expect.equals("P1", p1);
     Expect.equals("P2", p2);
   }
 
   foo("P1", "P2");
   bar("P1", "P2");
 }