Fails to check "Objects as maps"

My code:

```js
export type ContextTypes = {
query: { [string]: string },
};
```

It is built fine, but when I run on a browser, the error log below appears:

```
Warning: Failed prop type: propValue.hasOwnProperty is not a function
```

Reference: [Objects as maps in Flow Types](https://flow.org/en/docs/types/objects/#toc-objects-as-maps)

Start by reproducing the browser warning with the reported ContextTypes declaration and compare the plugin's generated PropTypes behavior for Flow's "Objects as maps" syntax. Use the linked Flow documentation as the type reference; done means this declaration builds and runs without the reported prop-type warning.

Written by the indexing model from the issue text.