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:     } else if (selector.isGetter()) {
        assert(arguments == null);
        if (callee.isFunction()) {
          FunctionTypeInformation functionInfo = info;
          functionInfo.canBeClosurized = true;
        }
        return;
  
negative.code: 
  class FieldTypeInformation extends TypeInformation {
    TypeMask type;
    Map<Element, int> callers = new Map<Element, int>();
    Map<Node, TypeMask> assignments = new Map<Node, TypeMask>();
    int analyzeCount = 0;
  
  