From the source code: flutterbincachedart-sdklibcollectioniterable.dart /// Operations on iterables with nullable elements. @Since(“3.0”) extension NullableIterableExtensions<T extends Object> on Iterable<T?> {   /// [Returns] The same elements as this iterable,   /// except that `null` values are omitted.   Iterable<T> get nonNulls => NonNullsIterable<T>(this); } dart:collection Source — changelog: https://github.com/dart-lang/sdk/blob/5d30f18892f1f825943a74e81ab02f27c2c6c26f/CHANGELOG.md#dartcollection Added extension members: nonNulls firstOrNull lastOrNull singleOrNull elementAtOrNull Obtained