supportsCubeTextures property
Whether a cube texture can be created and sampled.
Asked rather than assumed for the same reason as supportsMipmaps: no backend here reports it as a capability, so the honest answer is a probe or a constant, and which of the two it is belongs to the backend. A caller that gets false has to have something to fall back to — for the sky that is the procedural gradient, which is why the textured sky is an option on top of it rather than a replacement for it.
Implementation
@override
bool get supportsCubeTextures => true;