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chromium/third_party/blink/web_tests/external/wpt/workers/postMessage_block.https.html 

<!DOCTYPE html>
<meta charset=utf-8>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<body>
<script>
const t = async_test('postMessage and block');

const w = new Worker('support/postMessage_block_worker.js');

w.onmessage = t.step_func_done(() => {
  const a = new Int32Array(new SharedArrayBuffer(4));
  w.postMessage(a);
  while (Atomics.load(a, 0) === 0);
  assert_equals(Atomics.load(a, 0), 1);
});
</script>
</body>