Unused class declaration is not removed

If you minify the following functions only one of the unused class declarations is removed.

```js
function fn(base) {
class foo extends base {}
}
function fn2(base) {
class bar extends base.prop {}
}
```
Becomes:
```js
function fn(a){}
function fn2(a){class b extends a.prop{}}
```

Obviously this is a contrived example but in my application this is causing a large tree of unused classes to make it into the client bundle.

I'm just testing on the command line with `babel`:
```
cat example.js | babel --presets=babili
```