whereKey method
Allows us to find an entry based on key
Example:
people.whereKey((key) => key.length < 5);
// {John: 20, Mary: 21}
const Map<String, int> people = {'John': 20, 'Mary': 21, 'Peter':20};
Implementation
Map<K, V> whereKey(bool Function(K key) f) =>
{...where((key, value) => f(key))};