Avoid temporary Vectors for arrow function parameter lists

src/preparser.h

Index: src/preparser.h
diff --git a/src/preparser.h b/src/preparser.h
index 156fd64a2e5db9aa2ea3f7cf8bbe5f75ae97e366..6c6a63ade3d30ba7f5c55aee9aa3dd641f8c0c67 100644
--- a/src/preparser.h
+++ b/src/preparser.h
@@ -1249,11 +1249,13 @@ class PreParserTraits {
                              bool is_generator, bool* ok);
 
   // Utility functions
-  Vector<PreParserIdentifier> ParameterListFromExpression(
-      PreParserExpression expression, bool* ok) {
+  unsigned DeclareArrowParametersFromExpression(PreParserExpression expression,
+                                                PreParserScope* scope,
+                                                Scanner::Location* dupe_loc,
+                                                bool* ok) {
     // TODO(aperez): Detect duplicated identifiers in paramlists.
     *ok = expression.IsValidArrowParamList();
-    return Vector<PreParserIdentifier>::empty();
+    return 0;
   }
 
   static AstValueFactory* ast_value_factory() { return NULL; }
@@ -2436,16 +2438,6 @@ template <class Traits>
 typename ParserBase<Traits>::ExpressionT ParserBase<
     Traits>::ParseArrowFunctionLiteral(int start_pos, ExpressionT params_ast,
                                        bool* ok) {
-  typename Traits::Type::ParameterIdentifierVector params =
-      Traits::ParameterListFromExpression(params_ast, ok);
-
-  if (!*ok) {
-    ReportMessageAt(Scanner::Location(start_pos, scanner()->location().beg_pos),
-                    "malformed_arrow_function_parameter_list");
-    params.Dispose();
-    return this->EmptyExpression();
-  }
-
   // TODO(aperez): Change this to use ARROW_SCOPE
   typename Traits::Type::ScopePtr scope =
       this->NewScope(scope_, FUNCTION_SCOPE);
@@ -2453,30 +2445,25 @@ typename ParserBase<Traits>::ExpressionT ParserBase<
   FunctionState function_state(&function_state_, &scope_, &scope, zone(),
                                this->ast_value_factory());
   Scanner::Location dupe_error_loc = Scanner::Location::invalid();
+  unsigned num_params = Traits::DeclareArrowParametersFromExpression(
+      params_ast, scope_, &dupe_error_loc, ok);
+  if (!*ok) {
+    ReportMessageAt(Scanner::Location(start_pos, scanner()->location().beg_pos),
+                    "malformed_arrow_function_parameter_list");
+    return this->EmptyExpression();
+  }
 
-  if (params.length() > Code::kMaxArguments) {
+  if (num_params > Code::kMaxArguments) {
     ReportMessageAt(Scanner::Location(params_ast->position(), position()),
                     "too_many_parameters");
     *ok = false;
-    params.Dispose();
     return this->EmptyExpression();
   }
 
-  for (int i = 0; i < params.length(); ++i) {
-    const IdentifierT param_name = params.at(i)->raw_name();
-    if (!dupe_error_loc.IsValid() && scope_->IsDeclared(param_name)) {
-      int param_pos = params.at(i)->position();
-      dupe_error_loc =
-          Scanner::Location(param_pos, param_pos + param_name->length());
-    }
-    scope_->DeclareParameter(param_name, VAR);
-  }
-
   ExpressionT expression = ParseArrowFunctionLiteralBody(
-      &function_state, scope, params.length(), Scanner::Location::invalid(),
+      &function_state, scope, num_params, Scanner::Location::invalid(),
       dupe_error_loc, Scanner::Location::invalid(),
       FunctionLiteral::kNotParenthesized, start_pos, CHECK_OK);
-  params.Dispose();
   return expression;
 }