Invalid "Suspicious re-assignment of "exports" variable" warning

Compiling with ProcessCommonJSModules the example from https://github.com/google/closure-library/wiki/goog.module:-an-ES6-module-like-alternative-to-goog.provide:

```
goog.module('ns.Foo');
class Foo {}
exports = Foo;
```

yields (with today's tip):

```
Foo.js:5: WARNING - Suspicious re-assignment of "exports" variable. Did you actually intend to export something?
3| class Foo {}
4|
5| exports = Foo;
```

I'm pretty sure i'm not doing something wrong here. For now I'll suppress the warning.

Changing to `exports.Foo = Foo;` solves it, but changes the export pattern.

FYI @ChadKillingsworth