The RegisterStageExpanderForExtensionExpandable initializer may run before DocumentSourceExtensionExpandable::id is allocated. When it runs early, it registers the expander under kUnallocatedId (0) instead of the real ID. If desugaring can't find the expander for the real ID, stages are left as Expandable rather than Optimizable. We should add a prerequisite for "EndDocumentSourceIdAllocation" to ensure the expander is registered with the correct ID.