YMMV, but this worked for me to get Remix working on Node 12. I'm using Restify, but this is really just Node 12 polyfill stuff so it should work with other servers/adapters:

1. add fs.promises as a dependency
2. make a module promises-polyfill.js with the following:

   // this polyfills promise stuff for `fs` and even makes `require('fs/promises')` work
   require('fs.promises');
   
   // we also need to polyfill `Promise.all` for remix
   Promise.all = (promises) => {
     if (!promises.length) return Promise.resolve([]);
     if (promises.every(promise => !(promise instanceof Promise))) return new Promise((resolve) => setTimeout(() => resolve(promises)));
     let responses = [];
     let resolvedCount = 0;
     let rejected = false;
     return new Promise((resolve, reject) => {
       promises.forEach(async (promise, i) => {
         try {
           responses[i] = await promise;
           resolvedCount++;
           if (resolvedCount === promises.length) {
             resolve(responses);
           }
         } catch (e) {
           rejected = true;
           if (!rejected) reject(e);
         }
       });
     });
   };

3. set NODE_OPTIONS='--experimental-modules -r ./PATH/TO/YOUR/promises-polyfill' when running any remix commands (e.g. build or watch) as well as for your server process