[parser] Only parse async parenthesized arrow if current_token == ASYNC

Checking impl()->IsAsync(identifier) is insufficient since it could be
parenthesized. By checking the token in addition to IsIdentifier guarantees
that we've only seen the single token ASYNC.

Bug: chromium:898812
Change-Id: Id94dd607381050b4bd8cd6d8672a5d11256db7da
Reviewed-on: https://chromium-review.googlesource.com/c/1300134Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57013}

src/parsing/parser-base.h

@@ -3213,8 +3213,9 @@ ParserBase<Impl>::ParseLeftHandSideContinuation(ExpressionT result) {
         bool has_spread;
         ExpressionListT args(pointer_buffer());
         if (impl()->IsIdentifier(result) &&
-            impl()->IsAsync(impl()->AsIdentifier(result)) &&
+            scanner()->current_token() == Token::ASYNC &&
             !scanner()->HasLineTerminatorBeforeNext()) {
+          DCHECK(impl()->IsAsync(impl()->AsIdentifier(result)));
           ExpressionClassifier async_classifier(this);
           ParseArguments(&args, &has_spread, true);
           RETURN_IF_PARSE_ERROR;

test/mjsunit/regress/regress-898812.js

+// Copyright 2015 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)(a)=>{}", SyntaxError);