addThunksFrom method
Copies every deferred thunk (and any already-built memo) from other
into this registry without building the unresolved thunks. Used to
merge a pooled bundle's type lookup into a per-instance one (the lazy
counterpart of Map.addAll, which would force a build via forEach).
Implementation
void addThunksFrom(LazyBridgeRegistry<K> other) {
_thunks.addAll(other._thunks);
_memo.addAll(other._memo);
}