Fix async filtering (#3008)

Summary:
I relied too much on the types which in this case don't help at all.
Filtering on a promise will just automatically get all values to pass.

Pull Request resolved: https://github.com/facebook/flipper/pull/3008

Test Plan: `yarn build --mac` works again on the GH checkout.

Reviewed By: mweststrate

Differential Revision: D31991576

Pulled By: passy

fbshipit-source-id: f632d29ddfc09b7130b68b4b17264fd30e1969ce

desktop/scripts/get-app-watch-folders.ts

@@ -8,6 +8,7 @@
  */
 
 import fs from 'fs-extra';
+import pFilter from 'p-filter';
 import path from 'path';
 import {getWatchFolders} from 'flipper-pkg-lib';
 import {appDir, publicPluginsDir, fbPluginsDir} from './paths';
@@ -29,7 +30,8 @@ export default async function getAppWatchFolders() {
     ),
   );
   const watchFolders = ([] as string[]).concat(...getWatchFoldersResults);
-  return watchFolders
-    .filter((value, index, self) => self.indexOf(value) === index)
-    .filter(async (f) => fs.pathExists(f));
+  return pFilter(
+    watchFolders.filter((value, index, self) => self.indexOf(value) === index),
+    (f) => fs.pathExists(f),
+  );
 }