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chromium
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chromium/third_party/blink/web_tests/http/tests/security/resources/cross-frame-iframe-for-location-get-test.html 

<html>
<head>
    <script>
        window.location.existingCustomProperty = 1;
        window.location[1] = 1;
    </script>
</head>
<body>
</body>
</html>