There are still some issues to iron out with Animation.timeline being a read-write property, so we should follow what Firefox and Chrome do and make it read-only alone for now.
Cc'ing Chris Dumez and Sam Weinig for guidance about how to make a property read-only by default and read-write only if a given runtime flag is enabled. I don't think our code generator supports this at the moment.
<rdar://problem/58527432>
Created attachment 388971 [details] Patch
Created attachment 388976 [details] Patch
Committed r255234: <https://trac.webkit.org/changeset/255234>