code: public SoyType getOrCreateUnionType(Collection<SoyType> members) {
      SoyType type = UnionType.of(members);
      if (type.getKind() == SoyType.Kind.UNION) {
        type = unionTypes.intern((UnionType) type);
      }
      return type;
    }
docs: Factory function which creates a union type, given the member types. This folds identical union
  types together.
  
  @param members The members of the union.
  @return The union type.