read-only
Controls how one widget replaces another widget in the tree.
If the runtimeType and key properties of the two widgets are
operator==, respectively, then the new widget replaces the old widget by
updating the underlying element (i.e., by calling Element.update with the
new widget). Otherwise, the old element is removed from the tree, the new
widget is inflated into an element, and the new element is inserted into the
tree.