anchor.diff:    Map<Symbol,dynamic> get props => {#value: value};
   }
   
  -abstract class State {
  -  State clone();
  -}
  -
  +/// A Widget is a View that acts as a template. Its render() method typically
  +/// returns elements to be rendered
   abstract class Widget extends View {
     Map<Symbol, dynamic> _props;
     State _state, _nextState;
  
positive.code:   Map<Symbol, dynamic> get props => _props;
  }
  
  abstract class TagsApi {
    View Div({clazz, onClick, inner});
    View Span({clazz, onClick, inner});
  }
  
  class Tags implements TagsApi {
    noSuchMethod(Invocation inv) {
      if (inv.isMethod) {
  
negative.code:     return _nextState;
    }
  
    /// Sets the state to be rendered on the next update.
    /// Setting the nextState automatically marks the Widget as dirty.
    void set nextState(State s) {
      _nextState = s;
      updated.add(this);
  