Extending Kotlin's js wrapper classes

Using Kotlin/js, here's something that works:

fun main() {
    val canvas=document.getElementById("mainCanvas") as HTMLCanvasElement
    val ctx=canvas.getContext("2d") as CanvasRenderingContext2D
    val path=Path2D()
    path.rect(10.0,10.0,10.0,10.0)
    ctx.stroke(path)
}

Here's something that doesn't work:

fun main() {
    val canvas=document.getElementById("mainCanvas") as HTMLCanvasElement
    val ctx=canvas.getContext("2d") as CanvasRenderingContext2D
    val path=ExtraPath()
    path.rect(10.0,10.0,10.0,10.0)
    ctx.stroke(path)
}

class ExtraPath():Path2D(){
    fun extraFunction(){}
}

When it runs in the client, js complains that Path2D constructor: 'new' is required.

So what's the correct way to extend one of these js wrapper classes?