The issue this commit fixes occurs when we have an external interface
implemented by a Kotlin class, if that interface has methods with
varargs.
Kotlin functions expect varargs passed as arrays, but JavaScript code
may be unaware of this convention.
So, when generating a bridge for external interface method
implementaion, we insert some additional logic for extracting varargs
using the JavaScript `arguments` object.
A simplified example:
```kotlin
external interface Adder {
fun sum(vararg numbers: Int): Int
}
class AdderImpl: Adder {
override fun sum(vararg numbers: Int) = numbers.sum()
}
```
For `AdderImpl` we generate the following JS code:
```js
AdderImpl.prototype.sum_69wd7h_k$ = function (numbers) {
return sum(numbers);
};
AdderImpl.prototype.sum = function () {
var numbers = new Int32Array([].slice.call(arguments));
return this.sum_69wd7h_k$(numbers);
};
```
#KT-15223 Fixed
Sergej Jaskiewicz