Widget not going to bottom of screen in Flutter?

You will solve using Expanded widget.

        Expanded(
          child: Align(
            alignment: FractionalOffset.bottomCenter,
            child: MaterialButton(
              onPressed: () => {},
              child: Text(_register ? 'REGISTER' : 'LOGIN'),
            ),
          ),
        ),

There is another way to solve this:

children : [
           ///1
           Button(),

           Expanded(child:Container()),

           ///2
           Button(),
           ]

Logic: by expanded container will absorb the middle space in between the two buttons by which the second button will placed at bottom of the screen.

There is an easier way:

return Scaffold(     
  bottomNavigationBar: BottomAppBar(
    color: Colors.transparent,
    child: Text('bottom screen widget'),
    elevation: 0,
  ),
  body: Text('body')
  );