Instead of just ignoring the result of the second createPluginDescriptors call, can we avoid calling it twice in the first place? Something like this:

  let pluginsGen;
  return {
    options: optionsWithResolvedBrowserslistConfigFile(options, dirname),
    *plugins() {
      pluginsGen ??= createPluginDescriptors(
        options.plugins || [],
        dirname,
        alias,
      );
      return yield* pluginsGen;
    }
  };

If we cache the iterator instead, test will fail because the finished iterator always return undefined: https://github.com/JLHwung/babel/runs/7819160996?check_suite_focus=true

I pushed a commit to do the caching, so that now:

• The first call runs createPluginDescriptors, and after a while returns it
• The second call waits for the result of the first call, and returns it

Instead of

• The first call runs createPluginDescriptors, and returns the result of the createPluginDescriptors that finishes first
• The second call runs createPluginDescriptors, and returns the result of the createPluginDescriptors that finishes first

This avoids resolving the same plugins/presets multiple times, which is a costly operation because it involves the filesystem.