Returns an unmodifiable Map view of this.
The map uses the indices of this list as keys and the corresponding objects
as values. The Map.keys Iterable iterates the indices of this list
in numerical order.
List<String> words = ['fee', 'fi', 'fo', 'fum'];
Map<int, String> map = words.asMap();
map[0] + map[1]; // 'feefi';
map.keys.toList(); // [0, 1, 2, 3]
Source
Map<int, E> asMap();