The flow-upgrade tool deletes comments between class definition and props declaration

Bug: The `flow-upgrade` tool deletes comments between class definition and props declaration.
Version information:
```
grep flow-upgrade package.json
"flow-upgrade": "^1.0.4",
```

To reproduce this behaviour, create the following file:
```
/* @flow */
import React from 'react';
export class CviBiomarkerFactId extends React.PureComponent {
/** Some comment I want to keep here. */
props: {
id: string,
ui_label?: string,
};
}
```
then run `yarn create flow-upgrade`, which will result in the following file:
```
/* @flow */
import * as React from 'react';
export class CviBiomarkerFactId extends React.PureComponent<{
id: string,
ui_label?: string,
}> {}

```
Note that it ate the comment.