Deferred ListView.separator cleanups (#18720)

packages/flutter/lib/src/widgets/scroll_view.dart

@@ -452,7 +452,7 @@ abstract class BoxScrollView extends ScrollView {
 /// machinery can make use of the foreknowledge of the children's extent to save
 /// work, for example when the scroll position changes drastically.
 ///
-/// There are three options for constructing a [ListView]:
+/// There are four options for constructing a [ListView]:
 ///
 ///  1. The default constructor takes an explicit [List<Widget>] of children. This
 ///     constructor is appropriate for list views with a small number of
@@ -460,13 +460,18 @@ abstract class BoxScrollView extends ScrollView {
 ///     child that could possibly be displayed in the list view instead of just
 ///     those children that are actually visible.
 ///
-///  2. The [ListView.builder] takes an [IndexedWidgetBuilder], which builds the
-///     children on demand. This constructor is appropriate for list views with
-///     a large (or infinite) number of children because the builder is called
+///  2. The [ListView.builder] constructor takes an [IndexedWidgetBuilder], which
+///     builds the children on demand. This constructor is appropriate for list views
+///     with a large (or infinite) number of children because the builder is called
 ///     only for those children that are actually visible.
 ///
-///  3. The [ListView.custom] takes a [SliverChildDelegate], which provides the
-///     ability to customize additional aspects of the child model. For example,
+///  3. The [ListView.separated] constructor takes two [IndexedWidgetBuilder]s:
+///     `itemBuilder` builds child items on demand, and `separatorBuilder`
+///     similarly builds separator children which appear in between the child items.
+///     This constructor is appropriate for list views with a fixed number of children.
+///
+///  4. The [ListView.custom] constructor takes a [SliverChildDelegate], which provides
+///     the ability to customize additional aspects of the child model. For example,
 ///     a [SliverChildDelegate] can control the algorithm used to estimate the
 ///     size of children that are not actually visible.
 ///
@@ -749,7 +754,7 @@ class ListView extends BoxScrollView {
        childrenDelegate = new SliverChildBuilderDelegate(
          (BuildContext context, int index) {
            final int itemIndex = index ~/ 2;
-           return (index == 0 || index.isEven)
+           return index.isEven
              ? itemBuilder(context, itemIndex)
              : separatorBuilder(context, itemIndex);
          },