function assertListenerIsCalled(listener, numCount) {
  assertEquals(
      'Listeners is not called the correct number of times.', numCount,
      listener.getCallCount());
  listener[exports.ALREADY_CHECKED_PROP] = true;
  listener[exports.NUM_CALLED_PROP] = numCount;
}
Asserts that the given listener is called numCount number of times.
@param {!Function} listener The listener to check.
@param {number} numCount The number of times. See also the times()
    function below.
@suppress {strictMissingProperties} Added to tighten compiler checks