static resolveURL(path) {
    assert(this.#startCount > 0, "ensureStarted() should be called first");
    assert(!this.#startPromise, "ensureStarted() should have resolved");

    if (isNodeJS) {
      // Note: TestPdfsServer.ensureStarted() should be called first.
      return new URL(path, `http://127.0.0.1:${this.#webServer.port}/`);
    }
    // When "gulp server" is used, our URL looks like
    // http://localhost:8888/test/unit/unit_test.html
    // The PDFs are served from:
    // http://localhost:8888/test/pdfs/
    return new URL(TEST_PDFS_PATH + path, window.location);
  }
@param {string} path - path to file within test/unit/pdf/ (TEST_PDFS_PATH).
@returns {URL}