multipart parsing monkey-patch not working for installPolyfills()

[vekunz]

Describe the bug

In this pull request #5292 you "added" support for multipart parsing into undici. But it seems that this does not work, if I install polyfills with installPolyfills().

Reproduction

const formData = new FormData();
formData.set('email', '[email protected]');

const request = new Request('https://domain.de/my/path', {
  method: 'POST',
  headers: {},
  body: formData
});

await event.request.formData(); // <-- this failes

Logs

NotSupportedError: multipart/form-data not supported
 ❯ Request$2.formData ../../../../../../../../../C:/path/to/poject/node_modules/@sveltejs/kit/dist/node/polyfills.js:1921:13
 ❯ src/routes/signup/_index.spec.ts:97:26
     95|       };
     96| 
     97|       await event.request.formData();
       |                          ^

System Info

System:                                                    
    OS: Windows 10 10.0.19044                                
    CPU: (12) x64 Intel(R) Core(TM) i7-9850H CPU @ 2.60GHz   
    Memory: 13.04 GB / 31.79 GB                              
  Binaries:                                                  
    Node: 16.15.1 - C:\Program Files\nodejs\node.EXE         
    npm: 8.13.2 - C:\Program Files\nodejs\npm.CMD            
  Browsers:                                                  
    Chrome: 103.0.5060.66                                    
    Edge: Spartan (44.19041.1266.0), Chromium (103.0.1264.44)
    Internet Explorer: 11.0.19041.1566                       
  npmPackages:                                               
    @sveltejs/adapter-node: 1.0.0-next.78 => 1.0.0-next.78   
    @sveltejs/kit: 1.0.0-next.357 => 1.0.0-next.357          
    svelte: 3.48.0 => 3.48.0

Severity

blocking all usage of SvelteKit

Additional Information

Currently, this is one problem of at least two problems with undici, that block an update of SvelteKit (the other problem is that mswjs currently has no support for undici).

[Rich-Harris]

Please submit an actual reproduction

[vekunz]

I created a GitHub Repository to reproduce the issue https://github.com/vekunz/sveltekit-undici-formdata-issue.

While creating the repository I noticed that the error has changed in the latest release 1.0.0-beta.359.

SvelteKit 1.0.0-beta.358: NotSupportedError: multipart/form-data not supported
SvelteKit 1.0.0-beta.359: TypeError: Request.formData: undefined

I created two branches in the repository to demonstrate both issues.

[tradaonline]

I'm using FormData with Polyfill and have this problem too.