Rescope arrow-function parameter lists by moving the delta to the parameter scope

src/parsing/parser-base.h

Index: src/parsing/parser-base.h
diff --git a/src/parsing/parser-base.h b/src/parsing/parser-base.h
index 57e37c4fdf6617de73f69ca1c53f498858ec0985..a844fc0f21d500c866f289e0e16726adc5effdfa 100644
--- a/src/parsing/parser-base.h
+++ b/src/parsing/parser-base.h
@@ -2294,6 +2294,8 @@ ParserBase<Traits>::ParseAssignmentExpression(bool accept_IN,
   ExpressionClassifier arrow_formals_classifier(this,
                                                 classifier->duplicate_finder());
 
+  Scope::Snapshot scope_snapshot(scope());
+
   bool is_async = allow_harmony_async_await() && peek() == Token::ASYNC &&
                   !scanner()->HasAnyLineTerminatorAfterNext() &&
                   IsValidArrowFormalParametersStart(PeekAhead());
@@ -2343,8 +2345,8 @@ ParserBase<Traits>::ParseAssignmentExpression(bool accept_IN,
 
     scope->set_start_position(lhs_beg_pos);
     Scanner::Location duplicate_loc = Scanner::Location::invalid();
-    this->ParseArrowFunctionFormalParameterList(&parameters, expression, loc,
-                                                &duplicate_loc, CHECK_OK);
+    this->ParseArrowFunctionFormalParameterList(
+        &parameters, expression, loc, &duplicate_loc, scope_snapshot, CHECK_OK);
     if (duplicate_loc.IsValid()) {
       arrow_formals_classifier.RecordDuplicateFormalParameterError(
           duplicate_loc);