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.