Returns true if the collection contains an element equal to element.
This operation will check each element in order for being equal to
element, unless it has a more efficient way to find an element
equal to 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 contains uses.
Likewise the Iterable returned by a Map.keys call
should use the same equality that the Map uses for keys.
Source
bool contains(Object object) {
return _validKey(object) && _splay(object as dynamic/*=E*/) == 0;
}