anchor.diff:  
     @override
     Future<Rectangle> get offset async {
  -    var rect = await context.driver.execute(
  +    Map<String, num> rect = await context.driver.execute(
           '''return {
             left: arguments[0].offsetLeft,
             top: arguments[0].offsetTop,
  
positive.code:   WebDriverPageLoaderElement get globalContext => _globalContext;
  
    @override
    Future getInstance(Type type, [dynamic context]) async {
      if (context != null) {
        if (context is wd.SearchContext) {
          context = new WebDriverPageLoaderElement(context, this);
  
negative.code: 
    @override
    Future<Rectangle> getBoundingClientRect() async {
      Map<String, num> rect = await context.driver
          .execute('return arguments[0].getBoundingClientRect();', [context]);
      return new Rectangle<num>(rect['left'] as num, rect['top'] as num,
          rect['width'] as num, rect['height'] as num);
  