code: function getRelayHandleKey(
    handleName: string,
    key: ?string,
    fieldName: ?string,
  ): string {
    if (key && key !== DEFAULT_HANDLE_KEY) {
      return `__${key}_${handleName}`;
    }
  
    invariant(
      fieldName != null,
      'getRelayHandleKey: Expected either `fieldName` or `key` in `handle` to be provided',
    );
    return `__${fieldName}_${handleName}`;
  }
docs: @internal
  
  Helper to create a unique name for a handle field based on the handle name, handle key and
  source field.