// this assures that cancelling threads made in the same realm works.
// for cancelling across realms, another test would be needed.
svoid testThreadCancellation(double extraSleep default 0) {
  new Flag flag1;
  new Flag flag2;
  
  Thread t = startThread("testThreadCancellation", r {
    flag1.raise();
    try {
      busySleepSeconds(20.0);
    } finally {
      flag2.raise();
    }
  });
  flag1.waitUntilUp();
  sleepSeconds(extraSleep);
  assertFalse(flag2.isUp());
  long time = nanoTime();
  cancelThread(t);
  assertTrue("Thread cancellation works", flag2.waitUntilUp(2.0));
  time = nanoTime()-time;
  print("Local thread cancellation works. Took " + renderElapsedNanosPleasantly(time));
}