Change some dynamics to Object? and document slot better (#77026)

packages/flutter/lib/src/cupertino/text_selection_toolbar.dart

@@ -594,7 +594,7 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
     assert(!_forgottenChildren.contains(child));
     // Handle forgetting a child in children or in a slot.
     if (slotToChild.containsKey(child.slot)) {
-      final _CupertinoTextSelectionToolbarItemsSlot slot = child.slot as _CupertinoTextSelectionToolbarItemsSlot;
+      final _CupertinoTextSelectionToolbarItemsSlot slot = child.slot! as _CupertinoTextSelectionToolbarItemsSlot;
       slotToChild.remove(slot);
     } else {
       _forgottenChildren.add(child);