[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 217858524b56cb1230bd768bbc5f7ae0c8dc0f58..425341c2fbc18cf02d95bb800c0bccbc6688bdd1 100644
--- a/src/ast-value-factory.h
+++ b/src/ast-value-factory.h
@@ -171,6 +171,8 @@ 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) {