Update dart preamble code. (#87557)

DataTable and Button NNBD docs fixes

packages/flutter/lib/src/material/button_style.dart

@@ -35,7 +35,7 @@ import 'theme_data.dart';
 /// ```dart
 /// ElevatedButton(
 ///   style: ButtonStyle(
-///     backgroundColor: MaterialStateProperty.resolveWith<Color>(
+///     backgroundColor: MaterialStateProperty.resolveWith<Color?>(
 ///       (Set<MaterialState> states) {
 ///         if (states.contains(MaterialState.pressed))
 ///           return Theme.of(context).colorScheme.primary.withOpacity(0.5);

packages/flutter/lib/src/material/data_table.dart

@@ -167,7 +167,7 @@ class DataRow {
   ///
   /// ```dart
   /// DataRow(
-  ///   color: MaterialStateProperty.resolveWith<Color>((Set<MaterialState> states) {
+  ///   color: MaterialStateProperty.resolveWith<Color?>((Set<MaterialState> states) {
   ///     if (states.contains(MaterialState.selected))
   ///       return Theme.of(context).colorScheme.primary.withOpacity(0.08);
   ///     return null;  // Use the default value.
@@ -549,7 +549,7 @@ class DataTable extends StatelessWidget {
   /// {@template flutter.material.DataTable.dataRowColor}
   /// ```dart
   /// DataTable(
-  ///   dataRowColor: MaterialStateProperty.resolveWith<Color>((Set<MaterialState> states) {
+  ///   dataRowColor: MaterialStateProperty.resolveWith<Color?>((Set<MaterialState> states) {
   ///     if (states.contains(MaterialState.selected))
   ///       return Theme.of(context).colorScheme.primary.withOpacity(0.08);
   ///     return null;  // Use the default value.
@@ -597,7 +597,7 @@ class DataTable extends StatelessWidget {
   /// {@template flutter.material.DataTable.headingRowColor}
   /// ```dart
   /// DataTable(
-  ///   headingRowColor: MaterialStateProperty.resolveWith<Color>((Set<MaterialState> states) {
+  ///   headingRowColor: MaterialStateProperty.resolveWith<Color?>((Set<MaterialState> states) {
   ///     if (states.contains(MaterialState.hovered))
   ///       return Theme.of(context).colorScheme.primary.withOpacity(0.08);
   ///     return null;  // Use the default value.