anchor.diff:  
       observeTest('clear', () {
         list.clear();
  -      expect(list, []);
  +      expect(rol, []);
   
         performMicrotaskCheckpoint();
         expectChanges(records, [
  
positive.code:     observeTest('truncate and add new item', () {
        list.length = 1;
        list.add(42);
        expect(list, [1, 42]);
        performMicrotaskCheckpoint();
        expectChanges(changes, [
          _change(1, removedCount: 2, addedCount: 1)
  
negative.code: 
      observeTest('add changes length', () {
        list.add(4);
        expect(rol, [1, 2, 3, 4]);
        performMicrotaskCheckpoint();
        expectChanges(changes, [_lengthChange]);
      });
  
      observeTest('removeObject', () {
        list.remove(2);
        expect(rol, orderedEquals([1, 3]));
  
        performMicrotaskCheckpoint();
        expectChanges(changes, [_lengthChange]);
  