Unhandled exception on text nodes

[weizman]

1. Visit https://fasterthanlight.net/
2. Open devtools and run document.body.append('x')
3. The MObserver will catch the new text node and will try to execute "tachyon" in t.dataset but will fail because dataset is undefined for text nodes and in throws when right-hand is undefined

[weebney]

Good catch. I'll look into it this afternoon.