[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 acad434bc318438589eab139c5d92e91ec3ff312..22214d07f1580282e3a754007f2b4f42f10faa22 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -1313,7 +1313,7 @@ void ParserTraits::DeclareFormalParameter(
   bool is_duplicate = false;
   bool is_simple_name = pattern->IsVariableProxy();
   DCHECK(parser_->allow_harmony_destructuring() || is_simple_name);
-
+  if (is_rest) parsing_state->has_rest = true;
   const AstRawString* name = is_simple_name
                                  ? pattern->AsVariableProxy()->raw_name()
                                  : parser_->ast_value_factory()->empty_string();