whereKey method
Returns a new Map containing all the entries of this for which the key
satisfies test.
Example:
var map = {'a': 1, 'bb': 2, 'ccc': 3}
map.whereKey((key) => key.length > 1); // {'bb': 2, 'ccc': 3}
Implementation
Map<K, V> whereKey(bool Function(K) test) =>
    // Entries do not need to be cloned because they are const.
    Map.fromEntries(this.entries.where((entry) => test(entry.key)));