Gracefully handle proxies in AllCanWrite().

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

Index: test/mjsunit/harmony/regress/regress-576662.js
diff --git a/test/mjsunit/harmony/regress/regress-576662.js b/test/mjsunit/harmony/regress/regress-576662.js
new file mode 100644
index 0000000000000000000000000000000000000000..5541b79b5d1b538f5ae62a0530c30fd845bfe462
--- /dev/null
+++ b/test/mjsunit/harmony/regress/regress-576662.js
@@ -0,0 +1,11 @@
+// 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"));