Can't use a class's type as an `extends`

### Context

I noticed that the `flow-typed` project has a module definition for `express` which includes a naked top-level import of `{ Server } from 'http'` which AIUI is problematic. I figured I'd pitch in and try to pull the import into the `declare module` block, now that Flow supports that (as of v0.37). Doing this means pulling in some top-level `declare class`es as well, because those are where the import is used. The result is something like:

```javascript
declare module `express` {
declare class $Application {
...
}
...

declare module.exports: {
(): express$Application, // If you try to call like a function, it will use this signature
static: serveStatic, // `static` property on the function
Router: typeof express$Router, // `Router` property on the function
};
}
```

whereas originally there were global declarations along the lines of:

```javascript
declare class express$Application extends express$Router mixins events$EventEmitter {
...
}
```

Handy reference:

### Problem

The status quo declaration of `express` supports extending its declared classes by virtue of the fact that those classes are declared in the global scope (with a prefix), such as the following snippet from the `flow-typed` test for the module:

```javascript
declare class test_express$CustomApplication extends express$Application {
constructor(expressConstructor: () => express$Application): this;
...
}
```

But once `express$Application` becomes `$Application` and is moved into the `declare module` block, there doesn't seem to be a way to refer to it such that it can be named as a class to extend.

Because it's not exported as a value, saying `import { $Application } from 'express'` causes an error along the lines of:

```
2: import { $Application } from 'express';
^^^^^^^^^^^^ Named import from module `express`. This module has no named export called `$Application`.
```

But it's also not exported as a type, so saying `import type { $Application } from 'express'` causes pretty much the same error.

It looks like I can get the type to be exported by adding something like this to the module declaration: `declare type $ApplicationType = $Application`. If I do this, then I can import the type, but I can't actually use it in an `extends` clause in a class declaration:

```
4: declare class Foo extends $ApplicationType { }
^^^^^^^^^^^^^^^^ $ApplicationType. type referenced from value position
2: import type { $ApplicationType } from 'express';
^^^^^^^^^^^^^^^^ type $ApplicationType
```

### Transcript

I have a branch of `flow-typed` with a new version of `express` as outlined above, at . In this version (as of this writing), I have `declare class $ApplicationClass ...` in the module definition, which I try to `import` as a value in the tests.

When I run the `flow-typed` tests over it, here's the errors I get (with some duplication elided):

```
ERROR: express_v4.x.x/flow_v0.37.x-
* express_v4.x.x/flow_v0.37.x- (flow-v0.37.1): Unexpected Flow errors(2):
1-test_overrideUseMethodClassExtension.js:2
2: import express, { $ApplicationClass, $RequestClass, $ResponseClass, Router } from 'express'
^^^^^^^^^^^^^^^^^ Named import from module `express`. This module has no named export called `$ApplicationClass`.

1-test_overrideUseMethodClassExtension.js:2
2: import express, { $ApplicationClass, $RequestClass, $ResponseClass, Router } from 'express'
^^^^^^^^^^^^^ Named import from module `express`. This module has no named export called `$RequestClass`.

1-test_overrideUseMethodClassExtension.js:2
2: import express, { $ApplicationClass, $RequestClass, $ResponseClass, Router } from 'express'
^^^^^^^^^^^^^^ Named import from module `express`. This module has no named export called `$ResponseClass`.

1-test_overrideUseMethodClassExtension.js:57
57: // $ExpectError
^^^^^^^^^^^^^^^ Error suppressing comment. Unused suppression

1-test_overrideUseMethodClassExtension.js:59
59: // $ExpectError
^^^^^^^^^^^^^^^ Error suppressing comment. Unused suppression

1-test_overrideUseMethodClassExtension.js:63
63: // $ExpectError
^^^^^^^^^^^^^^^ Error suppressing comment. Unused suppression


Found 6 errors

null
```