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chromium/third_party/blink/web_tests/external/wpt/webmessaging/multi-globals/support/incumbent.html 

<!DOCTYPE html>
<meta charset="utf-8">
<title>Incumbent page used as a test helper</title>

<h1>Incumbent</h1>

<script>
"use strict";

window.createMessageChannel = () => {
  return new parent.frames[1].MessageChannel();
};
</script>