anchor.diff:  
     @override
     String putIfAbsent(String key, String ifAbsent()) {
  -    // TODO: implement putIfAbsent
  +    return "";
     }
   
     @override
     String remove(Object key) {
  -    // TODO: implement remove
  +    return "";
     }
   
     // TODO: implement values
  
positive.code: 
    @override
    bool containsKey(Object key) {
      // TODO: implement containsKey
    }
  
    @override
    bool containsValue(Object value) {
      // TODO: implement containsValue
    }
  
    @override
  
negative.code: 
    @override
    String operator [](Object key) {
      return "";
    }
  
    @override
  