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

复现 issue 中针对 warn(...arguments) 所展示的转换，然后将其与链接的 ember-simple-auth 报告进行比较。追踪导致 ES6 spread 语法保留在输出中的 Babel 插件行为，并验证生成的代码可以由 ember-cli-uglify@1 处理。

由索引模型根据 Issue 内容生成。