Safari errors in -prod mode, but not in Safari Technology preview and Chrome.

I'm using `ember-cli` 2.13.3 and `ember-cli-babili`.

When I launch `ember serve` everything is ok in both Safari latest and Safari Technology preview and Chrome.

But in production mode with: `ember serve -prod` Safari latest tells me this:

```
SyntaxError: Cannot declare a let variable twice: 'r'.
ReferenceError: Can't find variable: define
```

For Safari Technology preview and Chrome everything OK.

My ember-cli config > target.js is the default one:

```
module.exports = {
browsers: [
'ie 9',
'last 1 Chrome versions',
'last 1 Firefox versions',
'last 1 Safari versions'
]
};
```

I tried also with:

```
module.exports = {
browsers: [
'last 2 Chrome versions',
'last 2 Firefox versions',
'last 1 Safari versions',
'last 1 Edge versions'
]
};
```

or

```
module.exports = {
browsers: [
'last 2 Chrome versions',
'last 2 Firefox versions',
'last 2 Safari versions',
'last 2 Edge versions'
]
};
```