anchor.diff:  
       observeTest('add does not change existing items', () {
         list.add(4);
  -      expect(list, [1, 2, 3, 4]);
  +      expect(rol, [1, 2, 3, 4]);
         performMicrotaskCheckpoint();
         expectChanges(changes, []);
       });
   
       observeTest('[]= changes item', () {
         list[1] = 777;
  -      expect(list, [1, 777, 3]);
  +      expect(rol, [1, 777, 3]);
         performMicrotaskCheckpoint();
         expectChanges(changes, [_change(1, addedCount: 1, removedCount: 1)]);
       });
   
       observeTest('[]= on a different item does not fire change', () {
         list[2] = 9000;
  -      expect(list, [1, 2, 9000]);
  +      expect(rol, [1, 2, 9000]);
         performMicrotaskCheckpoint();
         expectChanges(changes, []);
       });
  
positive.code:     observeTest('add', () {
        list.add(5);
        list.add(6);
        expect(list, orderedEquals([1, 2, 3, 1, 3, 4, 5, 6]));
  
        performMicrotaskCheckpoint();
        expectChanges(records, [
  
negative.code:     observeTest('truncate and add same item', () {
        list.length = 1;
        list.add(2);
        expect(rol, [1, 2]);
        performMicrotaskCheckpoint();
        expectChanges(changes, [
          _change(1, removedCount: 2, addedCount: 1)
  