Gracefully handle proxies in AllCanWrite().

R=verwaest@chromium.org
BUG=chromium:576662,v8:1543
LOG=y

Review URL: https://codereview.chromium.org/1580723003

Cr-Commit-Position: refs/heads/master@{#33240}

src/objects.cc

@@ -1366,7 +1366,7 @@ Maybe<PropertyAttributes> JSObject::GetPropertyAttributesWithFailedAccessCheck(
 
 // static
 bool JSObject::AllCanWrite(LookupIterator* it) {
-  for (; it->IsFound(); it->Next()) {
+  for (; it->IsFound() && it->state() != LookupIterator::JSPROXY; it->Next()) {
     if (it->state() == LookupIterator::ACCESSOR) {
       Handle<Object> accessors = it->GetAccessors();
       if (accessors->IsAccessorInfo()) {

test/mjsunit/harmony/regress/regress-576662.js

+// Copyright 2016 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.
+
+// Flags: --harmony-proxies
+
+// https://code.google.com/p/chromium/issues/detail?id=576662 (simplified)
+
+Realm.create();
+this.__proto__ = new Proxy({},{});
+assertThrows(() => Realm.eval(1, "Realm.global(0).bla = 1"));