Use JavaScript runtime semantics when constant folding.

tests/language/argument_definition2_test.dart

Index: tests/language/argument_definition2_test.dart
diff --git a/tests/language/argument_definition2_test.dart b/tests/language/argument_definition2_test.dart
index d6abb8022393a239fb06af248cbf5d7166f6ee87..c59b061b03868311663b7a1dd4aa6845c6ac1777 100644
--- a/tests/language/argument_definition2_test.dart
+++ b/tests/language/argument_definition2_test.dart
@@ -41,6 +41,18 @@ class A {
   }
 }
 
+
+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() {
@@ -62,4 +74,7 @@ main() {
   closure = things[inscrutable(2)];
   closure(true, 54);
   closure(false);
+
+  test4(true, 54);
+  test4(false);
 }