code: function unionTypeAnnotation(types: Array<BabelAST>): BabelAST {
    invariant(
      types.length > 0,
      'RelayFlowBabelFactories: cannot create a union of 0 types',
    );
    return types.length === 1 ? types[0] : t.unionTypeAnnotation(types);
  }
docs: Create a union type if needed.
  
  TYPES[0] | TYPES[1] | ...