How to reproduce:
1. Open the attached project
2. Notice that the 'Assets' folder contains only a script that tries to override the TrackAsset.CreatePlayable method
3. Observe the Console window

Expected result: There are no Console errors and it is possible to override the TrackAsset.CreatePlayable method.
Actual result: The script causes a Console error "cannot override inherited member <...> because it is not marked virtual, abstract or override".

Reproducible with - 2018.1.0b10, 2018.1.9f2, 2018.2.0a2, 2018.2.15f1, 2018.3.0b9, 2019.1.0a8
Not reproducible with - 2017.4.14f1, 2018.1.0b9, 2018.2.0a1

Error thrown:
"error CS0506: `<...>.CreatePlayable(UnityEngine.Playables.PlayableGraph, UnityEngine.GameObject, UnityEngine.Timeline.TimelineClip)': cannot override inherited member `UnityEngine.Timeline.TrackAsset.CreatePlayable(UnityEngine.Playables.PlayableGraph, UnityEngine.GameObject, UnityEngine.Timeline.TimelineClip)' because it is not marked virtual, abstract or override"