anchor.diff:  
   // Iterable generating numbers in range [0..count).
   // May perform callback at some point underways.
  -class TestIterableBase<E> extends IterableBase<E> {
  +class TestIterableBase extends IterableBase<int> {
     final int length;
     final int count;
     // call [callback] if generating callbackIndex.
  
positive.code:   {
      // Change length of list after 20 additions.
      var l = [];
      var ci = new TestIterable<E>(257, 200, () {
        l.add("X");
      });
      Expect.throws(() {
  
negative.code: 
    {
      var l = [];
      var ci = new EfficientTestIterable(257, 257, 20, () {
        l.length = 0;
      });
      Expect.throws(() {
  