Perform escape analysis on imported types

Reference: [react-virtualized issue](https://github.com/bvaughn/react-virtualized/issues/1212)

This import appears in a [source file] which doesn't define components or composite types.

```js
import type WindowScroller from '../WindowScroller.js';
```

This line is in the output:

```
import { bpfrpt_proptype_WindowScroller } from '../WindowScroller.js';
```

As `bpfrpt_proptype_WindowScroller` is never referred to, the binding and import can be cleaned up.

[source file]: https://github.com/bvaughn/react-virtualized/blob/6faafb5c4398e502c568db73fc430e56e8434e81/source/WindowScroller/utils/onScroll.js