Returns true if the collection contains an element equal to
This operation will check each element in order for being equal to
element, unless it has a more efficient way to find an element
The equality used to determine whether
element is equal to an element of
the iterable defaults to the Object.== of the element.
Some types of iterable may have a different equality used for its elements.
For example, a Set may have a custom equality
(see Set.identity) that its
Iterable returned by a Map.keys call
should use the same equality that the
Map uses for keys.
bool contains(Object element) => _base.contains(element);