Dart APIunself_componentUnselfSliverNavigationBarheroTag heroTag property Null safety Object heroTag final Implementation final Object heroTag; unself_component unself_component UnselfSliverNavigationBar heroTag property UnselfSliverNavigationBar class Constructors UnselfSliverNavigationBar Properties automaticallyImplyLeading automaticallyImplyTitle backgroundColor border brightness hashCode heroTag key largeTitle leading middle opaque padding previousPageTitle runtimeType stretch trailing transitionBetweenRoutes Methods createElement createState debugDescribeChildren debugFillProperties noSuchMethod toDiagnosticsNode toString toStringDeep toStringShallow toStringShort Operators operator ==