Fix 'array_diff_assoc(): Argument #2 is not an array' error.

I've been having these `array_diff_assoc(): Argument #2 is not an array` errors on development since last `composer update`.

It seems that it's caused by the `$assets->map(...)->flatten()` call [here](https://github.com/laravel/framework/blob/master/src/Illuminate/Support/Collection.php#L264) because it returns another `Illuminate\Support\Collection` object and not an array.

Calling `toArray()` and the end of the call chain fixes the problem.