dead-code-elimiation doesn't remove unused top level code

For unused top level statements and expressions in module sources, dead-code-elimination does nothing, while in fact it is safe to remove them.

actual:
```js
function unused() {
}
class Unused {}
let notReferenced;
```

expected:
```js
// [empty]
```

Start by reproducing the issue with the provided module-source example and inspect the dead-code-elimination entry point for how it handles top-level statements and expressions. Done means unused top-level functions, classes, and variables are removed while the example produces an empty output.

Written by the indexing model from the issue text.