/**
* 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]
*/
Map<int, E> asMap();
Map類：
/**
* The map entries of [this].
*/
Iterable<MapEntry<K, V>> get entries;
