testAddCssRule() {
    // test that addCssRule correctly adds the rule to the style
    // sheet.
    const styleSheets = cssom.getAllCssStyleSheets();
    const styleSheet = styleSheets[3];
    const newCssRule = '.css-addCssRule { display: block; }';
    let rules = styleSheet.rules || styleSheet.cssRules;
    const origNumberOfRules = rules.length;

    cssom.addCssRule(styleSheet, newCssRule, 1);

    rules = styleSheet.rules || styleSheet.cssRules;
    const newNumberOfRules = rules.length;
    assertEquals(newNumberOfRules, origNumberOfRules + 1);

    // Remove the added rule so we don't mess up other tests.
    cssom.removeCssRule(styleSheet, 1);
  }
@suppress {missingProperties} cssRules not defined on StyleSheet