anchor.diff:  
   /** The [Iterator] returned by [EnumerateIterable.iterator]. */
   class EnumerateIterator<V> extends Iterator<IndexedValue<V>> {
  -  final Iterator<IndexedValue<V>> _iterator;
  +  final Iterator<V> _iterator;
     int _index = 0;
     IndexedValue<V> _current;
   
  
positive.code:   IndexedValue(this.index, this.value);
  }
  
  
  /**
   * An [Iterable] of [IndexedValue]s where the nth value holds the nth
   * element of [iterable] and its index. See [enumerate].
  
negative.code: 
    EnumerateIterable(this._iterable);
  
    Iterator<IndexedValue<V>> get iterator =>
        new EnumerateIterator<V>(_iterable.iterator);
  
    // Length related functions are independent of the mapping.
    int get length => _iterable.length;
  