ListBase<E> class Null safety

Abstract implementation of a list.

ListBase can be used as a base class for implementing the List interface.

This class implements all read operations using only the length and operator[] and members. It implements write operations using those and add, length= and operator[]= Classes using this base class should implement those five operations.

NOTICE: For backwards compatibility reasons, there is a default implementation of add which only works for lists with a nullable element type. For list with a non-nullable element type, the add method must be implemented.

NOTICE: Forwarding just the four length and [] read/write operations to a normal growable List (as created by a [] literal) will give very bad performance for add and addAll operations of ListBase. These operations are implemented by increasing the length of the list by one for each add operation, and repeatedly increasing the length of a growable list is not efficient. To avoid this, override 'add' and 'addAll' to also forward directly to the growable list, or, if possible, use DelegatingList from "package:collection/collection.dart" instead of a ListMixin.