constructor() {
    /** @protected */
    this._map = globalThisOrWindow[sym]
      ? globalThisOrWindow[sym]
      : (globalThisOrWindow[sym] = new Map());
  }
Allow compatibility with node-js (for ssr).
In the future, we can just use globalThis directly
(for now, we're backwards compatible with browsers that still only use window, since we don't know all contexts singleton-manager is used in).