Give insertBefore a Node or null as anchor

[EmilTholin]

referenceNode is not an optional parameter -- you must explicitly pass a Node or null. Failing to provide it or passing invalid values may behave differently in different browser versions.

This throws an error in IE9 and 10, so it might be worth defaulting the anchor component option to null:

mount_component(component, options.target, options.anchor || null);

Or possibly defaulting it at insert:

export function insert(target, node, anchor) {
  target.insertBefore(node, anchor || null);
}

I'm not sure what's best, so I thought I'd open an issue and discuss it before creating a PR.

[AlbertMarashi]

@Rich-Harris @EmilTholin having some errors with this

It's happening during hot-reload changes, but it only seems to be happening with the root element. Any idea why?

[AlbertMarashi]

I left some notes on https://discordapp.com/channels/457912077277855764/653341885674553345/706344295996588084

It's trying to insert a node at the target (which exists), but the anchor node (which is supposed to be in the target) is not in the target
with target.insertBefore(newNode, anchor || null)
anchor is removed from the target
so it's trying to inset a node before the target's anchor child, but the anchor isn't a child