addEntries method
The entries
will be added to the original map.
Note: entries
that already exist in the original map will overwrite
those of the original map, in place (keeping order).
Note: This will NOT sort anything.
Implementation
M<K, V> addEntries(Iterable<MapEntry<K, V>> entries) {
Map<K, V> map = Map.fromEntries(this.entries.followedBy(entries));
return MFlat<K, V>.unsafe(map);
}