addTags method
Add tags to objects in the registry.
Each element of the registry has a number of tags associated with it. A tag is either associated with an element or not, adding it more than once does not make any difference.
Tags are compared using Object.== equality.
Fails if any of the elements are not in the registry.
Implementation
Future addTags(Iterable<T> elements, Iterable<Object?> tags) {
var ids = elements.map(_getId).toList(growable: false);
return _addTags(ids, tags);
}