What is the best way to express an inner class?

I'm working with [openlayers](http://openlayers.org/) typing.
There is following inner class in code of _openlayers_.

``` javascript
// ol.Collection class
ol.Collection = function (/* ... parameters ... */) {
// ... class construction codes ...
}
// ... some methods

// ol.Collection.Error class
ol.Collection.Error = function (/* ... parameters ... */) {
// ... class construction codes ...
}
```

I try some typescript-like, C++-like, ... methods, (inner module, static inner class, ....) but no one works.
For example,

``` javascript
declare module 'ol' {
declare module 'Collection' {
declare class Error {
// ....
}
}
}
```

is rejected by flow.

Is there any clean choice for this case? If it is, could you give me some direction for typing this one?