Dart APIunself_componentUnselfNavigationBarheroTag heroTag property Null safety Object heroTag final Implementation final Object heroTag; unself_component unself_component UnselfNavigationBar heroTag property UnselfNavigationBar class Constructors UnselfNavigationBar Properties automaticallyImplyLeading automaticallyImplyMiddle backgroundColor border brightness hashCode heroTag key leading middle padding preferredSize previousPageTitle runtimeType trailing transitionBetweenRoutes Methods createElement createState debugDescribeChildren debugFillProperties noSuchMethod shouldFullyObstruct toDiagnosticsNode toString toStringDeep toStringShallow toStringShort Operators operator ==