anchor.diff:  }
   
   class PersistentVectorImpl<E> extends BaseVectorImpl<E> implements PersistentVector<E> {
  +  // cached hashCode.
  +  int _hashCode = null;
  +
  +
     factory PersistentVectorImpl.from(Iterable<E> values) {
       if (values.length == 0) {
         return new PersistentVectorImpl.empty();
  
positive.code:     this._size = 0;
      this._level = _SHIFT;
      this._root = this._tail = null;
      this._hashCode = null;
      this.__altered = true;
      return this;
    }
  
negative.code: 
    _get(int index) => (index >= 0 && index < this.length) ? this._array[index] : null;
  
    _VNode _removeAfter(Owner ownerID, int newSize) {
      var sizeIndex = (newSize - 1) & _MASK;
      if (sizeIndex >= this.length - 1) {
        return this;
      }
      var editable = _transientVNode(this, ownerID);
      editable._array.removeRange(sizeIndex + 1, editable.length);
      return editable;
  
    }
  