anchor.diff:                                 Selector selector,
                                  Element caller,
                                  Element callee) {
  +    typeInformationOf(callee).removeCall(caller);
       if (callee.isField()) {
         if (selector.isSetter()) {
           Map<Node, TypeMask> assignments = typeInformationOf(callee).assignments;
  
positive.code:   /**
     * Callers of an element.
     */
    Set<Element> get callers => null;
  
    /**
     * Number of times the element has been processed.
  
negative.code:   }
  
    Iterable<Element> getCallersOf(Element element) {
      return typeInformationOf(element).callers.keys;
    }
  
    /**
  