anchor.diff:    test('IsEmpty', () {
       Element root = insertTestDiv();
   
  -    List<Element> someChecked = document.querySelector('#someChecked').children;
  -
  -    List<Element> emptyDiv = document.querySelector('#emptyDiv').children;
  +    List<Element> someChecked =
  +        document.querySelector('#someChecked')!.children;
  +
  +    List<Element> emptyDiv = document.querySelector('#emptyDiv')!.children;
   
       expect(someChecked.length, 4);
       expect(emptyDiv.length, 0);
  
positive.code:   test('Last', () {
      Element root = insertTestDiv();
  
      List<Element> someChecked = document.querySelector('#someChecked').children;
  
      expect(someChecked.length, 4);
  
  
negative.code:     // Uses iterator.
      Element root = insertTestDiv();
  
      List<Element> eachChecked = document.querySelector('#allChecked')!.children;
  
      List<Element> someChecked =
          document.querySelector('#someChecked')!.children;
  
      List<Element> noneChecked =
          document.querySelector('#noneChecked')!.children;
  
      expect(eachChecked.length, 4);
      expect(someChecked.length, 4);
  