anchor.diff:     * not added if the [value] is equal to [def].
      */
     void addInt32(int field, int value, [int def]) {
  -    if (_currentVTableBuilder == null) {
  +    if (_currentVTable == null) {
         throw new StateError('Start a table before adding values.');
       }
       if (value != def) {
  
positive.code:    * Record the offset of the given [field].
     */
    void _trackField(int field) {
      _currentVTableBuilder.addField(field, _tail);
    }
  
    static void _setInt32AtTail(ByteData _buf, int tail, int x) {
  
negative.code: class Builder {
    final int initialSize;
  
    /**
     * The list of existing VTable(s).
     */
    final List<_VTable> _vTables = <_VTable>[];
  
    ByteData _buf;
  
    /**
  