Update to ListView Sample Code in API Docs (#29072)

* Updated ListView Sample code with more examples for different constructors and also to match asset diagrams.

* Fixed MIA semicolons.

* Code cleanup.

* Added context for ListView.builder example.

* Analyzer does not like const and static usages.

* Replaced the const declarations with final. The analyzer does not like the use of const here, at all.

* Fixed parameterized declarations.

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

@@ -604,17 +604,76 @@ abstract class BoxScrollView extends ScrollView {
 /// padding. To avoid this behavior, override with a zero [padding] property.
 ///
 /// {@tool sample}
+/// This example uses the default constructor for [ListView] which takes an
+/// explicit [List<Widget>] of children. This [ListView]'s children are made up
+/// of [Container]s with [Text].
 ///
-/// An infinite list of children:
+/// ```dart
+/// ListView(
+///   padding: const EdgeInsets.all(8.0),
+///   children: <Widget>[
+///     Container(
+///       height: 50,
+///       color: Colors.amber[600],
+///       child: const Center(child: Text('Entry A')),
+///     ),
+///     Container(
+///       height: 50,
+///       color: Colors.amber[500],
+///       child: const Center(child: Text('Entry B')),
+///     ),
+///     Container(
+///       height: 50,
+///       color: Colors.amber[100],
+///       child: const Center(child: Text('Entry C')),
+///     ),
+///   ],
+/// )
+/// ```
+/// {@end-tool}
+/// {@tool sample}
+/// This example mirrors the previous one, creating the same list using the
+/// [ListView.builder] constructor. Using the [IndexedWidgetBuilder], children
+/// are built lazily and can be infinite in number.
 ///
 /// ```dart
+/// final List<String> entries = <String>['A', 'B', 'C'];
+/// final List<int> colorCodes = <int>[600, 500, 100];
+///
 /// ListView.builder(
-///   padding: EdgeInsets.all(8.0),
-///   itemExtent: 20.0,
+///   padding: const EdgeInsets.all(8.0),
+///   itemCount: entries.length,
+///   itemBuilder: (BuildContext context, int index) {
+///     return Container(
+///       height: 50,
+///       color: Colors.amber[colorCodes[index]],
+///       child: Center(child: Text('Entry ${entries[index]}')),
+///     );
+///   }
+/// );
+/// ```
+/// {@end-tool}
+/// {@tool sample}
+/// This example continues to build from our the previous ones, creating a
+/// similar list using [ListView.separated]. Here, a [Divider] is used as a
+/// separator.
+///
+/// ```dart
+/// final List<String> entries = <String>['A', 'B', 'C'];
+/// final List<int> colorCodes = <int>[600, 500, 100];
+///
+/// ListView.separated(
+///   padding: const EdgeInsets.all(8.0),
+///   itemCount: entries.length,
 ///   itemBuilder: (BuildContext context, int index) {
-///     return Text('entry $index');
+///     return Container(
+///       height: 50,
+///       color: Colors.amber[colorCodes[index]],
+///       child: Center(child: Text('Entry ${entries[index]}')),
+///     );
 ///   },
-/// )
+///   separatorBuilder: (BuildContext context, int index) => const Divider(),
+/// );
 /// ```
 /// {@end-tool}
 ///