Partial revert of rest parameter desugaring.

src/parsing/parser.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_PARSING_PARSER_H_
 #define V8_PARSING_PARSER_H_
 
 #include "src/allocation.h"
 #include "src/ast/ast.h"
 #include "src/ast/scopes.h"
@@ skipping 1376 matching lines @@
     Expression* function, ZoneList<v8::internal::Expression*>* args, int pos) {
   return parser_->SpreadCallNew(function, args, pos);
 }
 
 
 void ParserTraits::AddFormalParameter(ParserFormalParameters* parameters,
                                       Expression* pattern,
                                       Expression* initializer,
                                       int initializer_end_position,
                                       bool is_rest) {
-  bool is_simple =
-      !is_rest && pattern->IsVariableProxy() && initializer == nullptr;
+  bool is_simple = pattern->IsVariableProxy() && initializer == nullptr;
   const AstRawString* name = is_simple
                                  ? pattern->AsVariableProxy()->raw_name()
                                  : parser_->ast_value_factory()->empty_string();
   parameters->params.Add(
       ParserFormalParameters::Parameter(name, pattern, initializer,
                                         initializer_end_position, is_rest),
       parameters->scope->zone());
 }
 
 
 void ParserTraits::DeclareFormalParameter(
     Scope* scope, const ParserFormalParameters::Parameter& parameter,
     ExpressionClassifier* classifier) {
   bool is_duplicate = false;
   bool is_simple = classifier->is_simple_parameter_list();
-  auto name = parameter.name;
-  auto mode = is_simple ? VAR : TEMPORARY;
+  auto name = is_simple || parameter.is_rest
+                  ? parameter.name
+                  : parser_->ast_value_factory()->empty_string();
+  auto mode = is_simple || parameter.is_rest ? VAR : TEMPORARY;
   if (!is_simple) scope->SetHasNonSimpleParameters();
   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());
   }
   if (is_sloppy(scope->language_mode())) {
     // TODO(sigurds) Mark every parameter as maybe assigned. This is a
@@ skipping 20 matching lines @@
 
 DoExpression* ParserTraits::ParseDoExpression(bool* ok) {
   return parser_->ParseDoExpression(ok);
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_PARSING_PARSER_H_