[es6] Refactor FormalParameter

src/preparser.cc

Index: src/preparser.cc
diff --git a/src/preparser.cc b/src/preparser.cc
index d60fa5659d8c460f004b62f9b97ec240b2268ab1..ec0ea2de3a04ba42bf5b32a54722420fb33de5d7 100644
--- a/src/preparser.cc
+++ b/src/preparser.cc
@@ -1053,11 +1053,11 @@ PreParser::Expression PreParser::ParseFunctionLiteral(
   int start_position = scanner()->location().beg_pos;
   function_scope->set_start_position(start_position);
   PreParserFormalParameters formals(nullptr);
-  int arity = ParseFormalParameterList(&formals, &formals_classifier, CHECK_OK);
+  ParseFormalParameterList(&formals, &formals_classifier, CHECK_OK);
   Expect(Token::RPAREN, CHECK_OK);
   int formals_end_position = scanner()->location().end_pos;
 
-  CheckArityRestrictions(arity, arity_restriction,
+  CheckArityRestrictions(formals.arity, arity_restriction,
                          formals.has_rest, start_position,
                          formals_end_position, CHECK_OK);