anchor.diff:    /**
      * Callers of an element.
      */
  -  Set<Element> get callers => null;
  +  Map<Element, int> get callers => null;
   
     /**
      * Number of times the element has been processed.
  
positive.code:     assert(caller.isImplementation);
      assert(callee.isImplementation);
      assert(isNotClosure(caller));
      Set<Element> callers = typeInformationOf(callee).callers;
      callers.add(caller);
    }
  
    bool addArguments(Node node,
  
negative.code:     // Bailout for closure calls. We're not tracking types of
      // closures.
      if (selector.isClosureCall()) return dynamicType;
      if (selector.isSetter() || selector.isIndexSet()) return dynamicType;
  
      TypeMask result;
      iterateOverElements(selector, (Element element) {
  