Use JavaScript runtime semantics when constant folding.

tests/language/argument_definition2_test.dart

 // Copyright (c) 2012, 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.
 
 test1(bool passed, [a = 42]) {
   if (passed) {
     Expect.equals(54, a);
     Expect.isTrue(?a);
   } else {
     Expect.equals(42, a);
@@ skipping 23 matching lines @@
       Expect.equals(54, a);
       Expect.isTrue(?a);
     } else {
       Expect.equals(42, a);
       Expect.isFalse(?a);
     }
     Expect.isTrue(?passed);
   }
 }
 
+
+test4(bool passed, [a]) {
+  if (passed) {
+    Expect.equals(54, a);
+    Expect.isTrue(?a);
+  } else {
+    Expect.equals(null, a);
+    Expect.isFalse(?a);
+  }
+  Expect.isTrue(?passed);
+}
+
 int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
 
 main() {
   test1(true, 54);
   test1(false);
   test2();
   new A().test3(true, 54);
   new A().test3(false);
 
   var things = [test1, test2, new A().test3];
 
   var closure = things[inscrutable(0)];
   closure(true, 54);
   closure(false);
 
   closure = things[inscrutable(1)];
   closure();
 
   closure = things[inscrutable(2)];
   closure(true, 54);
   closure(false);
+
+  test4(true, 54);
+  test4(false);
 }