<!DOCTYPE html>
<html>
<body>
<div id="console"></div>
<script>
document.__proto__.foo = "FAIL: Visible in isolated world.";
if (window.testRunner) {
  testRunner.dumpAsText();
  testRunner.evaluateScriptInIsolatedWorld(
    0,
    "document.getElementById('console').innerHTML = document.foo");
}
</script>
</body>
</html>