anchor.diff:    {
       // Change length of list after 20 additions.
       var l = [];
  -    var ci = new EfficientTestIterable<E>(257, 257, 20, () {
  +    var ci = new EfficientTestIterable(257, 257, 20, () {
         l.add("X");
       });
       Expect.throws(() {
  
positive.code: 
    {
      var l = [];
      var ci = new EfficientTestIterable<E>(257, 257, 20, () {
        l.length = 0;
      });
      Expect.throws(() {
  
negative.code:   {
      // Length 50, only 25 elements.
      var l = [];
      var ci = new EfficientTestIterable(500, 250);
      l.addAll(ci);
      Expect.listEquals(new List.generate(250, (x)=>x), l);
    }
  