[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..29dbb099547fe88073c659017b44ab8bb796cbfa 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -1320,7 +1320,8 @@ Expression* ParserTraits::SpreadCallNew(
 void ParserTraits::AddFormalParameter(
     ParserFormalParameters* parameters,
     Expression* pattern, Expression* initializer, bool is_rest) {
-  bool is_simple = pattern->IsVariableProxy() && initializer == nullptr;
+  bool is_simple =
+      !is_rest && pattern->IsVariableProxy() && initializer == nullptr;

[caitp (gmail), 2015/09/02 20:11:31]

This makes sure the empty string is used instead of the variable name, so that
the variable doesn't get declared twice. This seems to fix the bug in the tests.
I'm not sure if this code should be here, or maybe live somewhere else, but it
should be okay either way.

[rossberg, 2015/09/02 20:44:10]

Yes, this looks right to me.

   DCHECK(parser_->allow_harmony_destructuring() ||
          parser_->allow_harmony_rest_parameters() ||
          parser_->allow_harmony_default_parameters() || is_simple);
@@ -1338,10 +1339,8 @@ 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(