[es6] Re-implement rest parameters via desugaring.

src/parser.h

Index: src/parser.h
diff --git a/src/parser.h b/src/parser.h
index effa562e2bc9d25862bd37e8d57ac6ae9f87ee57..324ac93a724c11c8ec5d17d6e18949612de88c6a 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -1338,10 +1338,8 @@ void ParserTraits::DeclareFormalParameter(
     ExpressionClassifier* classifier) {
   bool is_duplicate = false;
   bool is_simple = classifier->is_simple_parameter_list();
-  // TODO(caitp): Remove special handling for rest once desugaring is in.
-  auto name = is_simple || parameter.is_rest
-      ? parameter.name : parser_->ast_value_factory()->empty_string();
-  auto mode = is_simple || parameter.is_rest ? VAR : TEMPORARY;
+  auto name = parameter.name;
+  auto mode = is_simple ? VAR : TEMPORARY;
   if (!is_simple) scope->SetHasNonSimpleParameters();
   bool is_optional = parameter.initializer != nullptr;
   Variable* var = scope->DeclareParameter(