Failure to render glTFs with animations

[cdata]

<model-viewer> cannot render the Khronos Brain Stem sample glTF. It fails with the following exception before anything is drawn:

The failure is on a call to update the model's skeleton: object.skeleton.update();.

Notably, this is a sample model with a built-in animation.

This bug was discovered while pursuing #360

[cdata]

This same issue occurs when trying to load the Khronos Cesium Man sample glTF. It's probably animation related.

[cdata]

Ditto: Khronos Monster sample glTF

[jsantell]

Looks like we have a SkinnedMesh without any skeleton