[parser] Reparse arrow functions with unidentified syntax errors in the correct scope

Bug: chromium:919710
Change-Id: I5a04e76fbc925a89b0ebe1916637f6ae5d109b24
Reviewed-on: https://chromium-review.googlesource.com/c/1400419Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58637}

src/parsing/parser-base.h

@@ -3952,6 +3952,8 @@ ParserBase<Impl>::ParseArrowFunctionLiteral(
         } else {
           // In case we did not sucessfully preparse the function because of an
           // unidentified error we do a full reparse to return the error.
+          // Parse again in the outer scope, since the language mode may change.
+          BlockState block_state(&scope_, scope()->outer_scope());
           ExpressionT expression = ParseConditionalExpression();
           DeclarationScope* function_scope = next_arrow_function_info_.scope;
           FunctionState function_state(&function_state_, &scope_,

test/mjsunit/regress/regress-919710.js

+// Copyright 2019 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("( let ) => { 'use strict';  let }", SyntaxError)