x/pkgsite: ioutil/Readfile does not run correctly in pkg.go.dev example.

[TimothyStiles]

Hello!

I think I've found a bug where example tests won't run properly in go.pkg.dev if they use any sort of file io.

What is the URL of the page with the issue?

https://pkg.go.dev/io/ioutil#example-ReadFile

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:92.0) Gecko/20100101 Firefox/92.0

Screenshot

What did you do?

I tried running this example from ioutil's docs. It returns the error after hitting the run button but renders the correct output by default.

What did you expect to see?

I expected to see the file's contents as printed output after hitting the run button

What did you see instead?

This:

It looks like the example's isolated environment doesn't have the file used in this test example. Which means that tests will run and pass locally and on CI/CD but not on pkg.go.dev itself. Is there any way to include the file in this rendered example's environment?

Thanks!
Tim

P.S

Looks like this is also the case for os.ReadFile.

[AlexanderYastrebov]

Similar to #32916 and related to #19825