[es6] Correct length for functions with default parameters

src/parser.h

Index: src/parser.h
diff --git a/src/parser.h b/src/parser.h
index 355778b042d3368b3f340cc58f08a357bc7879a4..8f0d440f3cabccfd785796ea02ca1770cd1cf3a0 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -1341,8 +1341,9 @@ void ParserTraits::DeclareFormalParameter(
   auto name = is_simple || parameter.is_rest
       ? parameter.name : parser_->ast_value_factory()->empty_string();
   auto mode = is_simple || parameter.is_rest ? VAR : TEMPORARY;
-  Variable* var =
-      scope->DeclareParameter(name, mode, parameter.is_rest, &is_duplicate);
+  bool is_optional = parameter.initializer != nullptr;
+  Variable* var = scope->DeclareParameter(
+      name, mode, is_optional, parameter.is_rest, &is_duplicate);
   if (is_duplicate) {
     classifier->RecordDuplicateFormalParameterError(
         parser_->scanner()->location());