title: How to center SliverGrid?
body: <p>I want to center a fixed width <code>GridView</code> in <code>CustomScrollView</code> horizontally.</p>
  <p>But I couldn't find a way to align as sliver. How can I do this?
  Maybe I can use <code>SliverToBoxAdapter</code> to use <code>Center</code> but is there no performance penalty?</p>
  <p>The following is pseudo code for this.</p>
  <pre class="lang-dart prettyprint-override"><code>CustomScrollView(
    slivers: [
      SliverToBoxAdapter(
        Center(child: Text('I can do it'))
      ),
      SliverAlign?(
        alignment: Alignment.topCenter,
        child: SliverSizedBox?(
          width: 600,
          sliver: SliverGrid.count(
            crossAxisCount: 2,
            mainAxisSpacing: 24,
            childAspectRatio: 0.9,
            crossAxisSpacing: 8,
            children: [
              Text('SliverAlign and SliverSizedBox does not exist')
            ],
      ))),
  ]);
  </code></pre>