map<K2, V2>  method 
- MapEntry<K2, V2> transform(- K,
- V
 
override
    Returns a new map where all entries of this map are transformed by
the given convert function.
Implementation
@override
Map<K2, V2> map<K2, V2>(MapEntry<K2, V2> Function(K, V) transform) =>
    _base.map((_, pair) => transform(pair.key, pair.value));