anchor.diff:  
       _readWriteGroup.remove();
       _readOnlyGroup.remove();
  -    _Streams.destroy(this);
  -
       _parentScope = null;
  -    broadcast(ScopeEvent.DESTROY);
     }
   }
   
  
positive.code:   }
  
    void destroy() {
      var prev = _prev;
      var next = _next;
      if (prev == null) {
  
negative.code:    */
    Watch watch(expression, ReactionFn reactionFn,
                {context, FilterMap filters, bool readOnly: false}) {
      assert(isAttached);
      assert(expression != null);
      AST ast;
      Watch watch;
  