Fix issue 18435.

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");
+}