Implementation of cast methods allows mutation without notifications.

[markegli]

Changes on ObservableMap and ObservableList objects created via cast or castFrom mutate the original collections but do not trigger notifications on the original collections.

final original = ObservableList<int>.from([1, 2, 3]);
original.changes.listen(print); // Listener never triggered.
final cast = original.cast<num>();
cast.add(4);
print(original); // Prints [1, 2, 3, 4].
await Future(() {}); // Flush any microtasks.