function assertNoOtherListenerIsCalled() {
  listeners.forEach(function(l, index) {
    if (!l[exports.ALREADY_CHECKED_PROP]) {
      assertEquals(
          'Listeners ' + index + ' is unexpectedly called.', 0,
          l.getCallCount());
    } else {
      assertEquals(
          'Listeners ' + index + ' is unexpectedly called.',
          l[exports.NUM_CALLED_PROP], l.getCallCount());
    }
  });
}
Asserts that no other listeners, other than those verified via
assertListenerIsCalled, have been called since the last
resetListeners().