warn() arguments are not being transpiled

```js
warn(...arguments);
```

is apparently being transpiled to

```js
(false && _ember.default.warn(...arguments));
```

but `...arguments` is ES6 code, which `ember-cli-uglify@1` is not able to handle.

see https://github.com/simplabs/ember-simple-auth/issues/1443

/cc @rwjblue