anchor.diff:  library range;
   import "dart:collection";
   class Range extends Object with IterableMixin<int> {
  -  Range(int this.start, [int this.stop, int this.step = 1]) {
  +  factory Range(int start, [int stop, int step = 1]) {
       if (stop == null ) { // reverse stop and start making start 0
         stop = start;
         start = 0;
  
positive.code:             step == other.step);
    }
  
    int start;
    int stop;
    final int step;
  }
  
  
negative.code:     if (step == 0) {
        throw new ArgumentError("step must not be 0");
      }
  
      return new Range._(start, stop, step);
    }
  
    Range._(this.start, this.stop, this.step);
  
    Iterator<int> get iterator {
        return new RangeIterator(start, stop, step);
  