Dart APIunself_componentUnSliverAppBaremoji emoji property Null safety String? emoji final Implementation final String? emoji; unself_component unself_component UnSliverAppBar emoji property UnSliverAppBar class Constructors UnSliverAppBar Properties description emoji hashCode implyLeading isCollapseCentered isImmersive key leading runtimeType title trailing Methods createElement createState debugDescribeChildren debugFillProperties noSuchMethod toDiagnosticsNode toString toStringDeep toStringShallow toStringShort Operators operator ==