[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 4a77d91b0ac859aeb6b938c40c36e4b00d5d37f1..9669ac4434abe6b91d68f28f31a823518fa1b64e 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -975,6 +975,7 @@ class Parser : public ParserBase<ParserTraits> {
     Scope* scope;
     VariableMode mode;
     bool is_const;
+    bool is_rest;
     bool needs_init;
     int declaration_pos;
     int initialization_pos;
@@ -993,6 +994,8 @@ class Parser : public ParserBase<ParserTraits> {
       Expression* pattern;
       int initializer_position;
       Expression* initializer;
+      bool is_rest = false;
+      int rest_literal_index = 0;
     };
 
     DeclarationParsingResult()
@@ -1314,9 +1317,9 @@ Expression* ParserTraits::SpreadCallNew(
 
 void ParserTraits::AddFormalParameter(
     ParserFormalParameters* parameters, Expression* pattern, bool is_rest) {
-  bool is_simple = pattern->IsVariableProxy();
-  DCHECK(parser_->allow_harmony_destructuring() ||
-         parser_->allow_harmony_rest_parameters() || is_simple);
+  bool is_simple = !is_rest && pattern->IsVariableProxy();
+  DCHECK((!is_rest || parser_->allow_harmony_rest_parameters()) ||
+         (!is_simple && parser_->allow_harmony_destructuring()) || is_simple);
   const AstRawString* name = is_simple
                                  ? pattern->AsVariableProxy()->raw_name()
                                  : parser_->ast_value_factory()->empty_string();
@@ -1330,10 +1333,9 @@ void ParserTraits::DeclareFormalParameter(
     Scope* scope, const ParserFormalParameters::Parameter& parameter,
     bool is_simple, ExpressionClassifier* classifier) {
   bool is_duplicate = false;
-  // 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 =
+      is_simple ? parameter.name : parser_->ast_value_factory()->empty_string();
+  auto mode = is_simple ? VAR : TEMPORARY;
   Variable* var =
       scope->DeclareParameter(name, mode, parameter.is_rest, &is_duplicate);
   if (is_duplicate) {