Repository: flutter
Issue: Row as leading/trailing in CupertinoSliverNavigationBar takes up the entire space covering the regular title
Labels: framework,f: cupertino,a: quality,P2,workaround available,team-design,triaged-design

## Steps to Reproduce
1. Use  CupertinoSliverNavigationBar
``` dart
class AppNavigationBar extends StatelessWidget {
  const AppNavigationBar(this._title, this._appLocalizations);

  final String _title;
  final AppLocalizations _appLocalizations;

  @override
  Widget build(BuildContext context) {
    return CupertinoSliverNavigationBar(
      border: Border.all(color: CupertinoColors.white),
      backgroundColor: CupertinoColors.white,
      largeTitle: Text(_title),
      middle: Text(_title),
      previousPageTitle: _appLocalizations.backButtonTitle,
      trailing: Row(
        mainAxisAlignment: MainAxisAlignment.end,
        crossAxisAlignment: CrossAxisAlignment.end,
        children: <Widget>[
          CupertinoButton(
              padding: const EdgeInsets.all(0),
              child: const Icon(
                CupertinoIcons.info,
                color: CupertinoColors.black,
                size: 32,
              ),
              onPressed: () {
                Navigator.push(
                    context,
                    PageRouteBuilder<CupertinoFullscreenDialogTransition>(
                        pageBuilder: (BuildContext context,
                                Animation<double> anim1,
                                Animation<double> anim2) =>
                            SettingsScreen(
                              localizations: _appLocalizations,
                            ),
                        transitionsBuilder: (BuildContext context,
                                Animation<double> animation,
                                Animation<double> secondaryAnimation,
                                Widget child) =>
                            CupertinoFullscreenDialogTransition(
                              animation: animation,
                              child: child,
                            )));
              }),
        ],
      ),
    );
  }
}
```
2 in  
``` dart
GestureDetector(
      onTap: () {
        _focus.unfocus();
      },
      child: CupertinoPageScaffold(
        child: CustomScrollView(
          physics: const AlwaysScrollableScrollPhysics(),
          controller: _scrollController,
          slivers: <Widget>[
            AppNavigationBar(_appLocalization.title, _appLocalization),
            CupertinoSliverRefreshControl(
              onRefresh: () {
                return Future<void>.delayed(Duration(seconds: 1))
                  ..then<void>((_) {
                    if (!mounted) {
                      return;
                    }
                    repopulateList();
                  });
              },
            ),
            SliverPersistentHeader(
              pinned: true,
              delegate: SliverAppBarDelegate(
                minHeight: Dimens.headerMinDimension,
                maxHeight: Dimens.headerMinDimension,
                child: _buildSearchField(context),
              ),
            ),
            SliverSafeArea(
              top: false, // Top safe area is consumed by the navigation bar.
              sliver: SliverFixedExtentList(
                itemExtent: Dimens.homeScreenItemExtent,
                delegate: SliverChildBuilderDelegate(
                  (BuildContext context, int index) {
                    if (index >= _loadedCarPlates.length) {
                      return _buildFooter();
                    } else {
                      return listItem()                      
                    }
                  },
                  childCount: n,
                ),
              ),
            ),
          ],
        ),
      ),
    )
``` 


<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
Flutter (Channel master, v1.5.9-pre.29, on Mac OS X 10.14.4 18E226, locale en-UA)
    • Flutter version 1.5.9-pre.29 at /Users/bogdanmigilev/Documents/flutter
    • Framework revision d121df9987 (3 hours ago), 2019-04-26 09:02:38 -0700
    • Engine revision 0b6a4be5f1
    • Dart version 2.3.0 (build 2.3.0-dev.0.3 7adad2a245)
```
## How it looks
![2019-04-26 22 21 10](https://user-images.githubusercontent.com/16256553/56928293-0d020c00-6adf-11e9-9485-8fa06cf2116b.gif)