Tree shaker defeated by `...args` and inner function

[live demo](https://closure-compiler.appspot.com/home#code%3D%252F%252F%2520%253D%253DClosureCompiler%253D%253D%250A%252F%252F%2520%2540compilation_level%2520ADVANCED_OPTIMIZATIONS%250A%252F%252F%2520%2540output_file_name%2520default.js%250A%252F%252F%2520%2540formatting%2520pretty_print%250A%252F%252F%2520%253D%253D%252FClosureCompiler%253D%253D%250A%250Afunction%2520makeDecorator()%2520%257B%250A%2520%2520%2520%2520function%2520Constructor(...args)%2520%257B%250A%2520%2520%2520%2520%2520%2520%250A%2520%2520%2520%2520%2520%2520function%2520NotReferenced()%2520%257B%257D%250A%2520%2520%2520%2520%250A%2520%2520%2520%2520%257D%250A%2520%2520%2520%2520return%2520Constructor%253B%250A%257D%250A%250Aconst%2520Bar%2520%253D%2520makeDecorator()%253B%250A%250ABar()%253B%250A)

Given:

```js
// ==ClosureCompiler==
// @compilation_level ADVANCED_OPTIMIZATIONS
// @output_file_name default.js
// @formatting pretty_print
// ==/ClosureCompiler==

function makeDecorator() {
function Constructor(...args) {

function NotReferenced() {}

}
return Constructor;
}

const Bar = makeDecorator();

Bar();
```

Produces:
```js
(function(b) {
for (var a = 0;a < arguments.length;++a) {
}
})();
```

Interestingly if we comment out `NotReferenced` it correctly tree-shakes