Raise SyntaxError on let [ starting an ExpressionStatement

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 ad336adcd6b2110e1fa7da79b927e15668c87ff4..bfde23f631f116f95fd31c7cb84826deddf83768 100644
--- a/src/parsing/parser-base.h
+++ b/src/parsing/parser-base.h
@@ -4980,6 +4980,12 @@ ParserBase<Impl>::ParseExpressionOrLabelledStatement(
       ReportUnexpectedToken(Next());
       *ok = false;
       return impl()->NullStatement();
+    case Token::LET:
+      if (PeekAhead() != Token::LBRACK) break;
+      impl()->ReportMessageAt(scanner()->peek_location(),
+                              MessageTemplate::kUnexpectedLexicalDeclaration);
+      *ok = false;
+      return impl()->NullStatement();
     default:
       break;
   }