How to give a gradient line in tab bar indicator

Question

I have a tab bar functionality in which I show a Donut chart and list of vehicles but I need to show which tab the user selected for that I have indicatorColor in TabBar but I need to fill with gradient line as shown in the image please help me out.

PS: If possible please let me know how to give theme color means primary color in main as a gradient???

return Scaffold(
    body: new DefaultTabController(
        length: 2,
        child: new Column(
          children: <Widget>[
            new Container(
              width: 1200.0,
              child: new Container(
                color: Colors.white,
                child: new TabBar(
                  labelColor: Colors.black,
                  tabs: [
                    Tab(
                      child: new Text("Visual",
                      style: new TextStyle(fontSize: 20.0)
                      ),
                    ),

                    Tab(
                      child: new Text("Tabular",
                      style: new TextStyle(fontSize: 20.0)), 
                    ),
                  ],
                ),
              ),
            ),
            new Expanded(
              child: new TabBarView(
                children: [
                  Tab(
                    child: new RefreshIndicator(
                      child: new Text('DONUT CHART'),
                      onRefresh: refreshList,
                      key: refreshKey1,
                    ),
                  ),
                  Tab(
                    child: new RefreshIndicator(
                      child: new Text('List of vehicles'),
                      onRefresh: refreshList,
                      key: refreshKey2,
                    ),
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );

Answer 1

Instead of changing core files here is a better answer that worked for me.

TabBar(
          controller: tabController,
          indicatorPadding: EdgeInsets.all(0.0),
          indicatorWeight: 4.0,
          labelPadding: EdgeInsets.only(left: 0.0, right: 0.0),
          indicator: ShapeDecoration(
              shape: UnderlineInputBorder(
                  borderSide: BorderSide(
                      color: Colors.transparent,
                      width: 4.0,
                      style: BorderStyle.solid)),
              gradient: LinearGradient(colors: [pinkLight, pinkDark])),
          tabs: <Widget>[
            Center(
              child: Container(
                alignment: Alignment.center,
                color: whiteColor,
                child: Text(
                  "Rating",
                  style: themeData.accentTextTheme.title,
                ),
              ),
            ),
            Container(
              alignment: Alignment.center,
              color: whiteColor,
              child: Text(
                "Category",
                style: themeData.accentTextTheme.title,
              ),
            ),
            Container(
              alignment: Alignment.center,
              color: whiteColor,
              child: Text(
                "Friend",
                style: themeData.accentTextTheme.title,
              ),
            ),
          ],
        ),

make indicator padding to zero, label padding to zero and indicatorWeight to 4.0 or your require size after that instead of using text in tabs use Container at top and give it color.