referenceNode argument to Node.insertBefore should be nullable

[zopsicle]

https://developer.mozilla.org/en-US/docs/Web/API/Node/insertBefore reads:

referenceNode The node before which newNode is inserted. If this is null, then newNode is inserted at the end of node's child nodes.

but Node.insertBefore requires a reference node to be provided.

Possible workaround:

case referenceNode of
  Just r -> Node.insertBefore newNode r this
  Nothing -> Node.appendChild newNode this

[garyb]

Yeah I think that workaround is preferable to matching the API exactly in this case, explicit is better than implicit.