putIfAbsent method
- K key,
- V ifAbsent()
override
    Throws an UnsupportedError; operations that change the map are disallowed.
Implementation
@override
V putIfAbsent(K key, V Function() ifAbsent) => _throw();Throws an UnsupportedError; operations that change the map are disallowed.
@override
V putIfAbsent(K key, V Function() ifAbsent) => _throw();