third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/foreign-fetch-helper-iframe.html - chromium/src - Git at Google

 <!DOCTYPE html>
 <meta charset=utf-8>
 <script>
 self.onmessage = e => {
     fetch(e.data.url)
       .then(response => response.text())
       .then(text => e.ports[0].postMessage('Success: ' + text))
       .catch(error => e.ports[0].postMessage('Error: ' + error));
   };
 </script>
	<!DOCTYPE html>
	<meta charset=utf-8>
	<script>
	self.onmessage = e => {
	fetch(e.data.url)
	.then(response => response.text())
	.then(text => e.ports[0].postMessage('Success: ' + text))
	.catch(error => e.ports[0].postMessage('Error: ' + error));
	};
	</script>