code: public JSType createUnionType(JSTypeNative... variants) {
      UnionTypeBuilder builder = UnionTypeBuilder.create(this);
      for (JSTypeNative typeId : variants) {
        builder.addAlternate(getNativeType(typeId));
      }
      return builder.build();
    }
docs: Creates a union type whose variants are the built-in types specified
  by the arguments.