_MergingListenable should use an Iterable #143664
Description
Use case
The _MergingListenable in change_notifier.dart uses a List to store its children. If the constructor is called with an Iterable for the children, it must be converted to a List unnecessarily.
Proposal
The type of _MergingListenable._children should be changed to Iterable<Listenable?>, as it is only used for iteration through for-loops.