Message formatting: handle unexpected case of failing property lookup.

R=cbruni@chromium.org
BUG=chromium:523308
LOG=N

Review URL: https://codereview.chromium.org/1314543004

Cr-Commit-Position: refs/heads/master@{#30330}

src/messages.cc

@@ -449,6 +449,7 @@ MaybeHandle<String> ErrorToStringHelper::Stringify(Isolate* isolate,
 bool ErrorToStringHelper::ShadowsInternalError(
     Isolate* isolate, LookupIterator* property_lookup,
     LookupIterator* internal_error_lookup) {
+  if (!property_lookup->IsFound()) return false;
   Handle<JSObject> holder = property_lookup->GetHolder<JSObject>();
   // It's fine if the property is defined on the error itself.
   if (holder.is_identical_to(property_lookup->GetReceiver())) return true;

test/mjsunit/regress/regress-crbug-523308.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.
+
+var error;
+try { reference_error(); } catch (e) { error = e; }
+toString = error.toString;
+error.__proto__ = [];
+assertEquals("ReferenceError: reference_error is not defined",
+             toString.call(error));