anchor.diff:    }
   
     dynamic eval(expression, [Map locals]) {
  +    assert(isAttached);
       assert(expression == null ||
              expression is String ||
              expression is Function);
  
positive.code: 
      _readWriteGroup.remove();
      _readOnlyGroup.remove();
      _Streams.destroy(this);
  
      _parentScope = null;
      broadcast(ScopeEvent.DESTROY);
    }
  }
  
  
negative.code:   void destroy() {}
  
    void _transitionState(String from, String to) {
      assert(isAttached);
      if (_state != from) throw "$_state already in progress can not enter $to.";
      _state = to;
    }
  