Use private like super keyword

[marcoNonvanilla]

The recently added super keyword is great to eliminate unnecessary boilerplate. I think the same could easily be done for private:

class ClassName {
   final int _myNum;
   final int _myPubNum;

  ClassName({required private.myNum, required this.myPubNum});
}

[eernstg]

I think this is covered by #2509 (search for 'super'). If you wish to recommend that we use a different syntax (such as private.myNum rather than _myNum, this._myNum, super._myNum) then it's probably better to comment on that issue than it is to create this new one.