Setting letterSpacing in a TextStyle makes text label in Tab off-center

gistfile1.txt

import 'package:flutter/material.dart';

void main() {
  runApp(new MaterialApp(
    home: new MyHomePage(),
  ));
}

class TabbedPage extends StatefulWidget {
  TabbedPage({ Key key, this.pageIndex, this.tabCount }) : super(key: key);
  final int pageIndex;
  final int tabCount;

  _TabbedPageState createState() => new _TabbedPageState();
}

class _TabbedPageState extends State<TabbedPage> with TickerProviderStateMixin {
  TabController _tabController;

  @override
  void initState() {
    _tabController = new TabController(
      length: widget.tabCount,
      vsync: this,
      initialIndex: PageStorage.of(context).readState(context) ?? 0,
    )..addListener(() {
      PageStorage.of(context).writeState(context, _tabController.index);
    });
    super.initState();
  }

  Widget build(BuildContext context) {
    return new Column(
      children: <Widget>[
        new Container(
          color: Theme.of(context).primaryColor,
          child: new TabBar(
            indicatorColor: Colors.red,
            indicatorWeight: 5.0,
            labelStyle: const TextStyle(
              letterSpacing: 5.0,
            ),
            indicatorPadding: const EdgeInsets.symmetric(horizontal: 12.0),
            key: new PageStorageKey<Type>(TabBar),
            controller: _tabController,
            isScrollable: true,
            tabs: new List<Tab>.generate(widget.tabCount, (int tabIndex) {
              return new Tab(
                text: 'AL',
              );
            }),
          ),
        ),
        new Expanded(
          child: new TabBarView(
            key: new PageStorageKey<Type>(TabBarView),
            controller: _tabController,
            children: new List<Widget>.generate(widget.tabCount, (int tabIndex) {
              return new Center(
                child: new Text('View ${widget.pageIndex}-${tabIndex}'),
              );
            }),
          ),
        ),
      ],
    );
  }
}

class MyHomePage extends StatefulWidget {
  _MyHomePageState createState() => new _MyHomePageState();
}

const List<int> tabCounts = const <int>[13, 17, 21];

class _MyHomePageState extends State<MyHomePage> with TickerProviderStateMixin {
  PageController _controller = new PageController();

  Widget build(BuildContext context) {
    return new PageView(
      controller: _controller,
      children: new List<Widget>.generate(tabCounts.length, (int index) {
        return new Scaffold(
          appBar: new AppBar(
            title: new Text('Page $index'),
          ),
          body: new TabbedPage(
            key: new PageStorageKey<int>(index),
            pageIndex: index,
            tabCount: tabCounts[index],
          ),
        );
      }),
    );
  }
}