How to remove extra padding around AppBar leading icon in Flutter

In my Flutter app, I have this AppBar

Widget setAppBar() {
  return AppBar(
    title: addAppBarTextWidget('Explore'),
    elevation: 0.0,
    leading: addLeadingIcon(),
    actions: <Widget>[
      addAppBarActionWidget(Constant.iconNotification, 22.0, 16.0, 8.0),
      addAppBarActionWidget(Constant.iconProfile, 30.0, 30.0, 15.0),
    ],
  );
}

Widget addLeadingIcon() {
  return Container(
    height: 25.0,
    width: 25.0,
    padding: EdgeInsets.zero,
    margin: EdgeInsets.zero,
    child: Stack(
      alignment: AlignmentDirectional.center,
      children: <Widget>[
        Image.asset(Constant.iconNotification, width: 25.0, height: 25.0),
        FlatButton(
          onPressed: () {
            onLeadingShowCategoryClick();
          },
        ),
      ],
    ),
  );
}

it looks like:

As you can see on the AppBar, there is some extra padding around the leading icon. How can I remove this extra padding.

You can't do this because it is a predefined widget. You can, however, do this:

appBar: AppBar(
  automaticallyImplyLeading: false, // Don't show the leading button
  title: Row(
    mainAxisAlignment: MainAxisAlignment.start,
    crossAxisAlignment: CrossAxisAlignment.center,
    children: <Widget>[
      IconButton(
        onPressed: () => Navigator.pop(context),
        icon: Icon(Icons.arrow_back, color: Colors.white),
      ),
      // Your widgets here
    ],
  ),
),

Where automaticallyImplyLeading: true hides the leading IconButton so you can add your own widgets.