anchor.diff:    {
       // Length 25, but 50 elements.
       var l = [];
  -    var ci = new EfficientTestIterable<E>(250, 500);
  +    var ci = new EfficientTestIterable(250, 500);
       l.addAll(ci);
       Expect.listEquals(new List.generate(500, (x)=>x), l);
     }
  
positive.code: 
    {
      var l = [];
      var ci = new EfficientTestIterable<E>(257, 257, 20, () {
        l.length = 0;
      });
      Expect.throws(() {
  
negative.code:   {
      // Change length of list after 20 additions.
      var l = [];
      var ci = new TestIterable(257, 200, () {
        l.add("X");
      });
      Expect.throws(() {
  