anchor.diff:  
     EnumerateIterable(this._iterable);
   
  -  Iterator<V> get iterator => new EnumerateIterator<V>(_iterable.iterator);
  +  Iterator<IndexedValue<V>> get iterator =>
  +      new EnumerateIterator<V>(_iterable.iterator);
   
     // Length related functions are independent of the mapping.
     int get length => _iterable.length;
  
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: 
  /** The [Iterator] returned by [EnumerateIterable.iterator]. */
  class EnumerateIterator<V> extends Iterator<IndexedValue<V>> {
    final Iterator<V> _iterator;
    int _index = 0;
    IndexedValue<V> _current;
  
  