anchor.diff:  
       observeTest('clear', () {
         list.clear();
  -      expect(list, []);
  +      expect(rol, []);
   
         performMicrotaskCheckpoint();
         expectChanges(records, [
  
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('removeRange changes length', () {
        list.add(4);
        list.removeRange(1, 3);
        expect(rol, [1, 4]);
        performMicrotaskCheckpoint();
        expectChanges(changes, [_lengthChange]);
      });
  
      observeTest('length= changes length', () {
        list.length = 5;
        expect(rol, [1, 2, 3, null, null]);
        performMicrotaskCheckpoint();
        expectChanges(changes, [_lengthChange]);
      });
  
      observeTest('[]= does not change length', () {
        list[2] = 9000;
        expect(rol, [1, 2, 9000]);
        performMicrotaskCheckpoint();
        expectChanges(changes, []);
      });
  
      observeTest('clear changes length', () {
        list.clear();
        expect(rol, []);
        performMicrotaskCheckpoint();
        expectChanges(changes, [_lengthChange]);
      });
  