[parser] Check if async function before throwing error

This changes the DCHECK (which could correctly fail) to be part of the
conditional that checks if we're in an async function.

Bug: chromium:751789
Change-Id: I3b8c1239ac93190055622c41fa1122e83b69d255
Reviewed-on: https://chromium-review.googlesource.com/607356Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47261}

src/parsing/parser-base.h

@@ -713,8 +713,7 @@ class ParserBase {
     }
 
     *ok = false;
-    if (current == Token::AWAIT) {
-      DCHECK(!is_async_function());
+    if (current == Token::AWAIT && !is_async_function()) {
       ReportMessageAt(current_location,
                       MessageTemplate::kAwaitNotInAsyncFunction, kSyntaxError);
       return;

test/mjsunit/regress/regress-751789.js

+// Copyright 2017 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(() => eval('async A=>{s.await i}'));