[es6] implement default/optional parameters (WIP / comments)

src/ast-value-factory.h

Index: src/ast-value-factory.h
diff --git a/src/ast-value-factory.h b/src/ast-value-factory.h
index cb69579314c63ca5e9b398d294d7a3ddab69eb72..7cdfd538a8aad490199b684c2c8d9a1ae0701550 100644
--- a/src/ast-value-factory.h
+++ b/src/ast-value-factory.h
@@ -171,6 +171,10 @@ class AstValue : public ZoneObject {
 
   void Internalize(Isolate* isolate);
 
+  bool IsUndefined() const {
+    return type_ == UNDEFINED;
+  }
+
   // Can be called after Internalize has been called.
   V8_INLINE Handle<Object> value() const {
     if (type_ == STRING) {