Method IsDirectory

tensorflow/core/platform/file_system_test.cc:268–273

Only allow for a single root directory.

 public:
  // Only allow for a single root directory.
  Status IsDirectory(const string& dirname) override {
    if (dirname == "." || dirname.empty()) {
      return Status::OK();
    }
    return Status(tensorflow::error::FAILED_PRECONDITION, "Not a dir");
  }

  // Simulating a FS with a root dir and a single file underneath it.
  Status GetChildren(const string& dir, std::vector<string>* result) override {