anchor.diff:   * Class that describes the structure of a table.
    */
   class _VTable {
  -  final List<int> fieldTails = <int>[];
  -  List<int> fieldOffsets;
  +  final Int32List _reusedFieldTails;
  +  final Int32List _reusedFieldOffsets;
  +
  +  /**
  +   * The number of fields in [_reusedFieldTails].
  +   */
  +  int _fieldCount = 0;
  +
  +  /**
  +   * The private copy of [_reusedFieldOffsets], which is made only when we
  +   * find that this table is unique.
  +   */
  +  Int32List _fieldOffsets;
   
     /**
      * The size of the table that uses this VTable.
  
positive.code:  */
  class Builder {
    final int initialSize;
  
    /**
     * The list of existing VTable(s).
  
negative.code:     assert(tail == null);
      assert(existing.tail != null);
      if (tableSize == existing.tableSize &&
          _fieldCount == existing._fieldCount) {
        for (int i = 0; i < _fieldCount; i++) {
          if (_reusedFieldOffsets[i] != existing._fieldOffsets[i]) {
            return false;
          }
        }
  