Move function length tracking from Scope to (Pre)?ParserFormalParameters.

src/ast/scopes.cc

Index: src/ast/scopes.cc
diff --git a/src/ast/scopes.cc b/src/ast/scopes.cc
index d9f34c894238ce934848e3636e7a361e0b602208..03d4613ffd26cb916d5f7f9edf0b0a4002749827 100644
--- a/src/ast/scopes.cc
+++ b/src/ast/scopes.cc
@@ -271,7 +271,6 @@ void DeclarationScope::SetDefaults() {
   function_ = nullptr;
   arguments_ = nullptr;
   this_function_ = nullptr;
-  arity_ = 0;
 }
 
 void Scope::SetDefaults() {
@@ -840,9 +839,6 @@ Variable* DeclarationScope::DeclareParameter(
     // TODO(wingo): Avoid O(n^2) check.
     *is_duplicate = IsDeclaredParameter(name);
   }
-  if (!is_optional && !is_rest && arity_ == params_.length()) {
-    ++arity_;
-  }
   has_rest_ = is_rest;
   params_.Add(var, zone());
   if (name == ast_value_factory->arguments_string()) {