CODE EXAMPLE FOR DART

dart map where

import 'package:collection/collection.dart';
...
Map<int, String> a = {1: "a", 2: "b", 3: "c", 4: "d", 5: "e", 6: "f"};

final b = Map.fromEntries(
    a.entries.map((entry) =>
        entry.key % 2 == 0 ? MapEntry(entry.key * 10, entry.value) : null)
    .whereNotNull()
);

print(b.keys.toList()); // prints [20, 40, 60]