anchor.diff:    SqlColumn alias(String alias) {
       SqlColumnImpl column = _cachedColumnAliases[alias];
       if (column == null) {
  -      column = nodeManager.registerNode(
  -          new SqlColumnImpl.aliased(alias, this, isPrimaryKey, table));
  +      column = nodeManager.registerNode(createColumnAlias(alias));
   
         if (isShared) {
           column = column.share();
  
positive.code:   }
  
    @override
    sql.SqlNode get as =>
        isAliased ? sql.as(target, name) : sql.as(this, qualifiedName);
  
    @override
    SqlColumn get autoAlias => preAlias("${table.name}_");
  
negative.code:   @override
    SqlColumnImpl createClone(bool freeze) =>
        new SqlColumnImpl.cloneFrom(this, freeze);
  
    SqlColumn createColumnAlias(String alias) =>
        new SqlColumnImpl.aliased(alias, this, isPrimaryKey, table);
  
    @override
    createNode() {
  