Properties from Object.assign(module.exports) escape type-checking

I'm compiling the following with `--process_common_js_modules`:
```
// a.js
const c = require("./c");
console.log(c.foo);

// c.js
module.exports = {};
Object.assign(module.exports, {
foo: 5,
});
```

and get an error:
```
a.js:11: WARNING - Property foo never defined on module$experimental$users$cyriln$js$c.default
console.log(c.foo);
^^^
```

even though the compiled output is functional:
```
// some $jscomp polyfills omitted
var module$experimental$users$cyriln$js$c$default = {};
Object.assign(module$experimental$users$cyriln$js$c$default, {foo:5});
console.log(module$experimental$users$cyriln$js$c$default.foo);
```

However, if the `Object.assign` is applied to a regular variable, typing works fine:
```
var c = {};
Object.assign(c, {
foo: 5,
});

module.exports = c;
```
and generates exactly same code.

Something is not right about processing `module.exports`.