Implement parsing of ES6 Rest Parameters

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index 2e2eea7fcd439efaae1a2599ac6c4a2b6201e7cb..08941f501f333511f612603e597e627327fc9373 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -3702,7 +3702,8 @@ FunctionLiteral* Parser::ParseFunctionLiteral(
       if (!reserved_error_loc.IsValid() && is_strict_reserved) {
         reserved_error_loc = scanner()->location();
       }
-      if (!dupe_error_loc.IsValid() && scope_->IsDeclared(param_name)) {
+      if (!dupe_error_loc.IsValid() &&
+          scope_->IsDeclaredParameter(param_name)) {
         duplicate_parameters = FunctionLiteral::kHasDuplicateParameters;
         dupe_error_loc = scanner()->location();
       }