Node.insertBefore() second parameter is not optional

[ntrrgc]

This is in the definition of a DOM Node, from lib.d.ts:

insertBefore(newChild: Node, refChild?: Node): Node;

According to it, refChild is optional, which is not true according to the MDN documentation and certainly causes a runtime error in Google Chrome.

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.