retainWhere method Null safety

void retainWhere(
  1. bool test(
    1. E element

Removes all objects from this list that fail to satisfy test.

An object o satisfies test if test(o) is true.

var numbers = ['one', 'two', 'three', 'four'];
numbers.retainWhere((item) => item.length == 3);
numbers.join(', '); // 'one, two'

The list must be growable.


void retainWhere(bool test(E element)) {
  throw new UnsupportedError("Cannot remove from immutable List.");