[parser] Restore RETURN_IF after export default var decl

Bug: chromium:900383, v8:8363, v8:7926
Change-Id: I6e3e38ee4cc986757926ef745d2e35865ba797a1
Reviewed-on: https://chromium-review.googlesource.com/c/1309633Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57158}

src/parsing/parser.cc

@@ -1211,6 +1211,7 @@ Statement* Parser::ParseExportDefault() {
       // no way of writing to it.
       Declaration* decl =
           DeclareVariable(local_name, VariableMode::kConst, pos);
+      RETURN_IF_PARSE_ERROR;
       decl->proxy()->var()->set_initializer_position(position());
 
       Assignment* assignment = factory()->NewAssignment(

test/message/regress/fail/regress-900383.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.
+//
+// MODULE
+
+export default x = 1;
+export default x = 1;

test/message/regress/fail/regress-900383.out

+*%(basename)s:8: SyntaxError: Identifier '*default*' has already been declared
+export default x = 1;
+                   ^
+SyntaxError: Identifier '*default*' has already been declared