expand<T> method Null safety

Expands each element of this Iterable into zero or more elements.

The resulting Iterable runs through the elements returned by f for each element of this, in iteration order.

The returned Iterable is lazy, and calls f for each element of this every time it's iterated.

Example:

var pairs = [[1, 2], [3, 4]];
var flattened = pairs.expand((pair) => pair).toList();
print(flattened); // => [1, 2, 3, 4];

var input = [1, 2, 3];
var duplicated = input.expand((i) => [i, i]).toList();
print(duplicated); // => [1, 1, 2, 2, 3, 3]