assertNotVisible = function(a, opt_b) {
  'use strict';
  _validateArguments(1, arguments);
  const element = nonCommentArg(1, 1, arguments);
  if (!element) {
    return;
  }

  _assert(
      commentArg(1, arguments), !goog.testing.style.isVisible(element) ||
          !goog.testing.style.hasVisibleDimensions(element),
      'Specified element should not be visible.');
}
The counter assertion of assertIsVisible().
@param {Element|string} a The element or optionally the comment string.
@param {Element=} opt_b The element when a comment string is present.