anchor.diff:    Element _child;
     bool _dirty = true;
   
  +  void mount(Element parent, dynamic slot) {
  +    super.mount(parent, slot);
  +    assert(_child == null);
  +    _rebuild();
  +    assert(_child != null);
  +  }
  +
  +  // This is also called for the first build
     void _rebuild() {
       assert(_debugLifecycleState == _ElementLifecycle.mounted);
       _dirty = false;
  
positive.code:     _child = _updateChild(_child, built, _slot);
    }
  
    void _rebuildIfNeeded() {
      if (_dirty)
        _rebuild();
    }
  
    void scheduleBuild() {
      assert(_debugLifecycleState == _ElementLifecycle.mounted);
      if (_dirty)
        return;
      _dirty = true;
      _buildScheduler.schedule(this);
      // TODO(abarth): Implement rebuilding.
    }
  
    void visitChildren(ElementVisitor visitor) {
  
negative.code:   }
  }
  
  /// Instantiation of StatefulComponent widgets.
  class StatefulComponentElement extends BuildableElement<StatefulComponent> {
    StatefulComponentElement(StatefulComponent widget)
      : _state = widget.createState(), super(widget) {
      assert(_state._config == widget);
      _state._element = this;
      _builder = _state.build;
    }
  