Syntax error when filename contains invalid character

I stumbled upon this when using it with next.js.

Next.js sometimes tries to compile pages with the suffix `?entry` which results in this error:

```
error in ./pages/foo?entry

Syntax Error: Unexpected token, expected ; (6:11)
4 | import Link from "next/dist/lib/link.js";
5 |
> 6 | var indexJs?entry = function indexJsEntry() {
| ^
7 | return React.createElement(
8 | "div",
9 | {
```

(`/pages/foo` is a directory with an `index.js` file)

As you see, the function name is correct but the var name still contains the `?`.

I think the ideal behavior would be to strip the `?entry` completely, so the function name would correctly resolve to the parent directory name.