assertIsVisible = function(a, opt_b) {
  'use strict';
  _validateArguments(1, arguments);
  const element = nonCommentArg(1, 1, arguments);

  _assert(
      commentArg(1, arguments), goog.testing.style.isVisible(element) &&
          goog.testing.style.hasVisibleDimensions(element),
      'Specified element should be visible.');
}
Asserts that an element has:
  1 - a CSS rendering the makes the element visible.
  2 - a non-zero width and height.
@param {Element|string} a The element or optionally the comment string.
@param {Element=} opt_b The element when a comment string is present.