Fix let pattern error accumulation

src/expression-classifier.h

Index: src/expression-classifier.h
diff --git a/src/expression-classifier.h b/src/expression-classifier.h
index bd9ba7064b62f12c10ccad3652e3d1116c4e558d..7392a7add8bd3c6ff63a8aba043c66cbfdb52ce8 100644
--- a/src/expression-classifier.h
+++ b/src/expression-classifier.h
@@ -264,6 +264,8 @@ class ExpressionClassifier {
       if (errors & StrongModeFormalParametersProduction)
         strong_mode_formal_parameter_error_ =
             inner.strong_mode_formal_parameter_error_;
+      if (errors & LetPatternProduction)
+        let_pattern_error_ = inner.let_pattern_error_;
     }
 
     // As an exception to the above, the result continues to be a valid arrow