capacity method Null safety

Implementation

int capacity() {
  var exception = _getException();

  int result = _dll_handle.lookupFunction<_DynamicTemplateIndex_capacity_c, _DynamicTemplateIndex_capacity_dart>(
      _c_namespace + "DynamicTemplateIndex_capacity")(_impl, exception);

  checkException(exception, _dll_handle);

  return result;
}