[parser] Only validate async params of valid arrow functions

Bug: chromium:898936
Change-Id: I2c8d8212fa97436e32628d4030d36ce457346c1f
Reviewed-on: https://chromium-review.googlesource.com/c/1300133Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57014}

src/parsing/parser-base.h

@@ -946,9 +946,10 @@ class ParserBase {
       }
     } else if (!classifier()->is_valid_arrow_formal_parameters()) {
       ReportClassifierError(classifier()->arrow_formal_parameters_error());
+    } else {
+      DCHECK_IMPLIES(is_async,
+                     classifier()->is_valid_async_arrow_formal_parameters());
     }
-    DCHECK_IMPLIES(is_async,
-                   classifier()->is_valid_async_arrow_formal_parameters());
   }
 
   void ValidateLetPattern() {

test/mjsunit/regress/regress-898936.js

+// Copyright 2018 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+assertThrows("async(...x=e)()=>");