IE11 - 'Symbol' is undefined when trying to use network

Hi,

In validating another issue https://github.com/almende/vis/issues/1911

discovered that snapshot in develop 4.18 works fine in Edge/Chrome/Firefox, but throws an error in IE11 when trying to instantiate vis Network

'Symbol' is undefined is the error

Plunker created to reproduce the error https://plnkr.co/edit/MX1agpoRYgYbgC23NAhl?p=preview