"line number not found in file stdio" when running "code"

After #4351, we have:

```m2
i1 : code(() -> return)
code(() -> return)

o1 = stdio:1:5-1:18: --source code:
code(() -> return)
stdio:2:4:(3):[1]: error: line number 1 not found in file stdio
```

Note that the two lines need to be entered together at the same prompt (e.g., copied and pasted) for this to fire.

Reproduce the error by entering the two shown lines together at one prompt, then inspect the code-handling path associated with #4351. Compare the behavior with the open pull request #4444; done means this input no longer reports that line number 1 is missing.

Written by the indexing model from the issue text.