Removes all objects from this list that fail to satisfy test.
An object o satisfies test if test(o) is true.
List<String> numbers = ['one', 'two', 'three', 'four'];
numbers.retainWhere((item) => item.length == 3);
numbers.join(', '); // 'one, two'
Throws an UnsupportedError if this is a fixed-length list.
Source
void retainWhere(bool test(E element)) {
_filter(this, test, true);
}